feat(perf): Remove inc_rc/dec_rc instructions that follow a removed load in mem2reg

[vezenovm]

Description

Problem*

Resolves #6088 (comment)

Summary*

If we have have an inc_rc/dec_rc instruction in mem2reg, we can check whether the previously instruction was a removed load. If it was, we can safely remove the inc_rc/dec_rc instruction.

Additional Context

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: fcaebedac4e48ca256af5fc8466ea4cfb98e9df8, compared to commit: 65a461f3f9cdd3a3fad0212ee3423e417cd29170

🧾 Summary (10% most significant diffs)

Program	Brillig opcodes (+/-)	%
nested_array_dynamic	-3 ✅	-0.11%
reference_only_used_as_alias	-6 ✅	-2.34%

---

Full diff report 👇

Program	Brillig opcodes (+/-)	%
hashmap	30,598 (-3)	-0.01%
uhashmap	25,476 (-15)	-0.06%
nested_array_dynamic	2,826 (-3)	-0.11%
reference_only_used_as_alias	250 (-6)	-2.34%

[vezenovm]

Closing as this was an experiment to improve a regression in #6088 which has been turned to a draft. Either way, we should keep mem2reg as only removing loads and stores and any inc_rc/dec_rc removals should come in other passes.