chore(ssa): Skip array_set pass for Brillig functions

[aakoshh]

Description

Problem*

Resolves #6486

Summary*

The PR changes the array_set pass to skip Brilling functions completely and remove all code related to is_brillig_runtime as well.

Additional Context

This is undoing the changes made in #6463 to fix a Brillig bug that made arrays shared via function parameters with the caller mutable. Apparently ACIR never had a problem with this, and not being able to do that optimisation makes it less efficient.

I suppose we could make this conditional on the already present is_brillig_runtime flag, however it was decided that the array_set pass was only ever meant to be used by ACIR, and Brillig should use refcounts exclusively to decide whether to mutate or not.

Documentation*

Check one:

• No documentation needed.
• Documentation included in this PR.
• [For Experimental Features] Documentation to be submitted in a separate PR.

PR Checklist*

• I have tested the changes locally.
• I have formatted the changes with Prettier and/or cargo fmt on default settings.

[github-actions]

Changes to Brillig bytecode sizes

Generated at commit: d8065ad5a1a6e0fb6588407883ada9d63ff7381f, compared to commit: 0fc0c53ec183890370c69aa4148952b3123cb055

🧾 Summary (10% most significant diffs)

Program	Brillig opcodes (+/-)	%
brillig_array_to_slice	+24 ❌	+26.67%
reference_only_used_as_alias	+33 ❌	+13.31%
pedersen_hash	+24 ❌	+6.70%
regression_struct_array_conditional	+30 ❌	+5.73%
merkle_insert	+36 ❌	+4.63%
pedersen_check	+27 ❌	+3.62%
brillig_pedersen	+27 ❌	+3.62%

---

Full diff report 👇

Program	Brillig opcodes (+/-)	%
brillig_array_to_slice	114 (+24)	+26.67%
reference_only_used_as_alias	281 (+33)	+13.31%
pedersen_hash	382 (+24)	+6.70%
regression_struct_array_conditional	554 (+30)	+5.73%
merkle_insert	814 (+36)	+4.63%
pedersen_check	772 (+27)	+3.62%
brillig_pedersen	772 (+27)	+3.62%
simple_shield	897 (+27)	+3.10%
array_to_slice	1,043 (+30)	+2.96%
schnorr	1,492 (+36)	+2.47%
fold_complex_outputs	573 (+12)	+2.14%
import	149 (+3)	+2.05%
array_sort	302 (+6)	+2.03%
sha2_byte	3,388 (+66)	+1.99%
brillig_rc_regression_6123	175 (+3)	+1.74%
hashmap	26,656 (+411)	+1.57%
nested_array_dynamic	2,181 (+33)	+1.54%
pedersen_commitment	200 (+3)	+1.52%
brillig_cow_regression	2,410 (+30)	+1.26%
databus_two_calldata	270 (+3)	+1.12%
nested_array_in_slice	1,202 (+12)	+1.01%
array_dynamic	320 (+3)	+0.95%
conditional_1	1,337 (+12)	+0.91%
regression_capacity_tracker	231 (+2)	+0.87%
regression	729 (+6)	+0.83%
slice_dynamic_index	2,736 (+22)	+0.81%
keccak256	1,778 (+12)	+0.68%
brillig_keccak	1,778 (+12)	+0.68%
uhashmap	16,310 (+102)	+0.63%
array_dynamic_nested_blackbox_input	994 (+6)	+0.61%
array_dynamic_blackbox_input	1,176 (+6)	+0.51%
6	1,367 (+6)	+0.44%
sha256	2,053 (+9)	+0.44%
sha256_var_witness_const_regression	1,403 (+6)	+0.43%
conditional_regression_short_circuit	1,437 (+6)	+0.42%
fold_numeric_generic_poseidon	770 (+3)	+0.39%
no_predicates_numeric_generic_poseidon	770 (+3)	+0.39%
brillig_sha256	796 (+3)	+0.38%
regression_4449	859 (+3)	+0.35%
sha256_var_padding_regression	5,248 (+18)	+0.34%
ram_blowup_regression	1,009 (+3)	+0.30%
ecdsa_secp256k1	1,013 (+3)	+0.30%
sha256_var_size_regression	2,046 (+6)	+0.29%
strings	1,042 (+3)	+0.29%
sha256_regression	7,272 (+18)	+0.25%
poseidonsponge_x5_254	4,502 (+6)	+0.13%
regression_5252	4,879 (+6)	+0.12%
poseidon_bn254_hash_width_3	5,692 (+6)	+0.11%
poseidon_bn254_hash	5,692 (+6)	+0.11%
u128	2,911 (+3)	+0.10%
eddsa	10,894 (+9)	+0.08%

[github-actions]

Changes to circuit sizes

Generated at commit: d8065ad5a1a6e0fb6588407883ada9d63ff7381f, compared to commit: 0fc0c53ec183890370c69aa4148952b3123cb055

🧾 Summary (10% most significant diffs)

Program	ACIR opcodes (+/-)	%	Circuit size (+/-)	%
array_dynamic_nested_blackbox_input	-28 ✅	-10.18%	-92 ✅	-1.24%
array_dynamic_main_output	-11 ✅	-25.00%	-42 ✅	-19.00%

---

Full diff report 👇

Program	ACIR opcodes (+/-)	%	Circuit size (+/-)	%
nested_array_dynamic	3,166 (-29)	-0.91%	12,930 (-11)	-0.09%
nested_dyn_array_regression_5782	32 (-3)	-8.57%	2,859 (-4)	-0.14%
array_dynamic	102 (-6)	-5.56%	3,714 (-17)	-0.46%
array_dynamic_nested_blackbox_input	247 (-28)	-10.18%	7,315 (-92)	-1.24%
array_dynamic_main_output	33 (-11)	-25.00%	179 (-42)	-19.00%

[github-actions]

Changes to number of Brillig opcodes executed

Generated at commit: d8065ad5a1a6e0fb6588407883ada9d63ff7381f, compared to commit: 0fc0c53ec183890370c69aa4148952b3123cb055

🧾 Summary (10% most significant diffs)

Program	Brillig opcodes (+/-)	%
reference_only_used_as_alias	+163 ❌	+66.53%
regression_capacity_tracker	+336 ❌	+58.95%
brillig_array_to_slice	+31 ❌	+37.80%
regression_struct_array_conditional	+270 ❌	+20.87%
pedersen_hash	+99 ❌	+17.22%
merkle_insert	+486 ❌	+14.48%

---

Full diff report 👇

Program	Brillig opcodes (+/-)	%
reference_only_used_as_alias	408 (+163)	+66.53%
regression_capacity_tracker	906 (+336)	+58.95%
brillig_array_to_slice	113 (+31)	+37.80%
regression_struct_array_conditional	1,564 (+270)	+20.87%
pedersen_hash	674 (+99)	+17.22%
merkle_insert	3,842 (+486)	+14.48%
simple_shield	2,885 (+306)	+11.87%
sha2_byte	52,231 (+5,049)	+10.70%
brillig_pedersen	1,252 (+117)	+10.31%
pedersen_check	1,252 (+117)	+10.31%
array_sort	538 (+36)	+7.17%
pedersen_commitment	296 (+18)	+6.47%
import	153 (+9)	+6.25%
array_to_slice	2,475 (+133)	+5.68%
fold_complex_outputs	889 (+36)	+4.22%
hashmap	58,421 (+2,079)	+3.69%
brillig_rc_regression_6123	289 (+9)	+3.21%
slice_dynamic_index	4,355 (+132)	+3.13%
array_dynamic_blackbox_input	19,955 (+594)	+3.07%
schnorr	10,396 (+216)	+2.12%
uhashmap	153,618 (+2,796)	+1.85%
databus_two_calldata	500 (+9)	+1.83%
nested_array_dynamic	3,116 (+54)	+1.76%
nested_array_in_slice	1,469 (+24)	+1.66%
array_dynamic_nested_blackbox_input	4,629 (+45)	+0.98%
regression	2,703 (+18)	+0.67%
poseidon_bn254_hash	169,552 (+1,053)	+0.62%
poseidon_bn254_hash_width_3	169,552 (+1,053)	+0.62%
conditional_1	5,860 (+36)	+0.62%
regression_5252	951,206 (+5,400)	+0.57%
poseidonsponge_x5_254	190,917 (+1,080)	+0.57%
strings	1,897 (+9)	+0.48%
regression_4449	219,762 (+630)	+0.29%
u128	25,428 (+72)	+0.28%
sha256_var_witness_const_regression	7,036 (+18)	+0.26%
sha256	10,844 (+27)	+0.25%
6	7,357 (+18)	+0.25%
conditional_regression_short_circuit	7,432 (+18)	+0.24%
eddsa	737,279 (+1,620)	+0.22%
brillig_sha256	4,200 (+9)	+0.21%
fold_numeric_generic_poseidon	5,239 (+9)	+0.17%
no_predicates_numeric_generic_poseidon	5,239 (+9)	+0.17%
ecdsa_secp256k1	7,075 (+9)	+0.13%
sha256_var_size_regression	18,331 (+18)	+0.10%
brillig_keccak	37,099 (+36)	+0.10%
keccak256	37,099 (+36)	+0.10%
brillig_cow_regression	583,839 (+252)	+0.04%
sha256_regression	126,254 (+45)	+0.04%
sha256_var_padding_regression	234,837 (+54)	+0.02%
ram_blowup_regression	877,404 (+9)	+0.00%