Add a simple simplify-cfg optimization for assembly

[vaivaswatha]

Description

When compiling code such as

fn main() {
     while true {}
}

we would end up with unreachable code in the assembly:

.program:
.2                                      ; --- start of function: main_0 ---
pusha .2                                ; save all regs
move $$locbase $sp                      ; save locals base register for main_0
cfei i0                                 ; allocate 0 bytes for locals and 0 slots for call arguments.
move $r28 $$reta                        ; save reta
move $r29 $$retv                        ; save retv
.6
ji  .7
.7
ji  .7
.3
cfsi i0                                 ; free 0 bytes for locals and 0 slots for extra call arguments.
move $$reta $r28                        ; restore reta
popa .2                                 ; restore all regs
jmp $$reta                              ; return from call

;; --- Data ---
.data:

Here the label .3 and what follows is unreachable because the program gets stuck in an infinite loops with label .7. This means that the instruction move $r28 $$reta is dead and hence removed by ASM DCE.

Our verifier on the other hand will see a use of $r28 in the unreachable code. But with the definition having been eliminated, it will now flag an error.

The verifier is unaware of reachability. Making the verifier more accurate is probably an overkill. So we workaround this by adding an unreachable code eliminator optimization in the assembly stage. While it has its own benefits (of removing unreachable code), it'll also make the verifier happy as-is. The algorithm is linear in the size of the code.

Closes #6174.

[github-actions]

Benchmark for aa659f2

Click to view benchmark

Test	Base	PR	%
code_action	5.0±0.10ms	5.3±0.04ms	+6.00%
code_lens	281.4±13.21ns	281.3±4.93ns	-0.04%
compile	2.5±0.02s	2.6±0.03s	+4.00%
completion	4.6±0.01ms	4.7±0.04ms	+2.17%
did_change_with_caching	2.4±0.04s	2.5±0.03s	+4.17%
document_symbol	922.0±21.46µs	873.3±20.52µs	-5.28%
format	75.5±1.12ms	75.1±0.97ms	-0.53%
goto_definition	337.8±5.63µs	335.4±6.46µs	-0.71%
highlight	8.7±0.21ms	9.0±0.02ms	+3.45%
hover	496.6±13.07µs	486.4±5.30µs	-2.05%
idents_at_position	119.4±0.34µs	118.7±0.52µs	-0.59%
inlay_hints	632.9±12.69µs	638.2±5.87µs	+0.84%
on_enter	478.3±18.20ns	467.4±9.10ns	-2.28%
parent_decl_at_position	3.6±0.04ms	3.7±0.05ms	+2.78%
prepare_rename	337.1±9.79µs	335.2±2.99µs	-0.56%
rename	8.9±0.16ms	9.3±0.17ms	+4.49%
semantic_tokens	1219.4±31.11µs	1245.8±12.04µs	+2.16%
token_at_position	341.5±2.35µs	350.2±1.95µs	+2.55%
tokens_at_position	3.6±0.03ms	3.7±0.03ms	+2.78%
tokens_for_file	394.9±1.91µs	402.2±3.66µs	+1.85%
traverse	37.4±1.02ms	38.4±0.92ms	+2.67%

[github-actions]

Benchmark for d0f0cec

Click to view benchmark

Test	Base	PR	%
code_action	5.1±0.14ms	5.2±0.07ms	+1.96%
code_lens	282.9±14.70ns	280.3±8.61ns	-0.92%
compile	2.5±0.02s	2.5±0.05s	0.00%
completion	4.6±0.12ms	4.7±0.05ms	+2.17%
did_change_with_caching	2.5±0.02s	2.5±0.04s	0.00%
document_symbol	941.8±32.67µs	916.0±43.04µs	-2.74%
format	73.0±0.83ms	73.4±0.60ms	+0.55%
goto_definition	347.9±10.94µs	344.4±25.21µs	-1.01%
highlight	8.7±0.22ms	9.0±0.11ms	+3.45%
hover	491.8±7.42µs	492.9±5.29µs	+0.22%
idents_at_position	118.8±0.28µs	118.9±0.28µs	+0.08%
inlay_hints	682.5±26.38µs	644.6±19.06µs	-5.55%
on_enter	467.3±12.05ns	461.3±7.87ns	-1.28%
parent_decl_at_position	3.6±0.02ms	3.7±0.03ms	+2.78%
prepare_rename	340.1±5.07µs	341.1±6.13µs	+0.29%
rename	8.9±0.17ms	9.3±0.12ms	+4.49%
semantic_tokens	1240.4±17.63µs	1260.3±13.73µs	+1.60%
token_at_position	337.9±2.31µs	335.6±2.83µs	-0.68%
tokens_at_position	3.5±0.05ms	3.7±0.04ms	+5.71%
tokens_for_file	397.6±4.06µs	397.7±3.85µs	+0.03%
traverse	38.1±0.85ms	37.6±0.98ms	-1.31%

[ironcev]

Awesome! Super simple solution 😄

Can you please also just enable the two disabled tests:
should_pass/language/break_and_continue_block_ret
should_pass/language/abort_control_flow_good

[github-actions]

Benchmark for 5d5df40

Click to view benchmark

Test	Base	PR	%
code_action	5.0±0.06ms	5.2±0.02ms	+4.00%
code_lens	283.9±9.25ns	279.8±4.29ns	-1.44%
compile	2.6±0.03s	2.5±0.02s	-3.85%
completion	4.7±0.07ms	4.8±0.03ms	+2.13%
did_change_with_caching	2.5±0.03s	2.5±0.02s	0.00%
document_symbol	869.1±27.10µs	884.5±22.62µs	+1.77%
format	73.9±1.02ms	74.0±0.81ms	+0.14%
goto_definition	376.3±7.21µs	402.4±5.87µs	+6.94%
highlight	8.7±0.16ms	9.1±0.06ms	+4.60%
hover	524.9±9.76µs	552.9±2.65µs	+5.33%
idents_at_position	119.3±0.91µs	118.9±0.88µs	-0.34%
inlay_hints	612.9±25.08µs	642.3±18.68µs	+4.80%
on_enter	479.5±23.47ns	468.0±9.82ns	-2.40%
parent_decl_at_position	3.6±0.10ms	3.7±0.04ms	+2.78%
prepare_rename	374.2±5.47µs	406.3±10.57µs	+8.58%
rename	9.0±0.43ms	9.4±0.24ms	+4.44%
semantic_tokens	1261.7±14.19µs	1245.0±7.53µs	-1.32%
token_at_position	334.2±2.22µs	336.6±7.20µs	+0.72%
tokens_at_position	3.5±0.03ms	3.7±0.02ms	+5.71%
tokens_for_file	398.1±2.57µs	406.9±3.78µs	+2.21%
traverse	38.7±0.81ms	38.2±0.89ms	-1.29%