Add Strength Reduction and Dead Code Elimination to AST Optimizer

[Psykii22]

This PR adds two new AST optimizer passes:

• Strength Reduction: Optimizes computationally heavy expressions
  without side effects. Specifically:

  • Eliminates division x / 2 in favor of multiplication x * 0.5.
  • Transforms exponents x ** 2 into x * x.
    Both optimizations use std::mem::replace instead of clone to avoid redundant memory allocations.

• Dead Code Elimination: Removes unreachable if, while, and for
  branches/loops when the condition is a known boolean literal (after constant folding).
  handles JavaScript hoisting semantics by bailing out if the removed
  body contains var declarations, function declarations, or generator/async variants.
  also, prevents side-effect vulnerabilities in for loops by
  refusing to eliminate loops that contain an initialization statement. like for(let i=doAuth();false;i++)

Also bounds all optimizer pass loops with MAX_PASS_ITERATIONS to prevent
infinite loops.

[github-actions]

Test262 conformance changes

Test result	main count	PR count	difference
Total	52,963	52,963	0
Passed	49,713	49,713	0
Ignored	2,262	2,262	0
Failed	988	988	0
Panics	0	0	0
Conformance	93.86%	93.86%	0.00%

Tested main commit: 5994303fbdc61f8cfa941a244d98ce6bfbd3ec5b
Tested PR commit: e73970b53e2d2d8c619fe5937828937e17002e1e
Compare commits: 5994303...e73970b

[codecov]

Codecov Report

❌ Patch coverage is 56.00000% with 66 lines in your changes missing coverage. Please review.
✅ Project coverage is 57.98%. Comparing base (6ddc2b4) to head (e73970b).
⚠️ Report is 804 commits behind head on main.

Files with missing lines	Patch %	Lines
...re/engine/src/optimizer/pass/strength_reduction.rs	38.77%	30 Missing ⚠️
...engine/src/optimizer/pass/dead_code_elimination.rs	50.87%	28 Missing ⚠️
core/engine/src/optimizer/mod.rs	81.81%	8 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #4758       +/-   ##
===========================================
+ Coverage   47.24%   57.98%   +10.74%     
===========================================
  Files         476      559       +83     
  Lines       46892    61208    +14316     
===========================================
+ Hits        22154    35493    +13339     
- Misses      24738    25715      +977     

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[jedel1043]

This is very cool, thank you!

By the way, did you have the opportunity to test if this increases performance in the V8 benchmarks?

[Psykii22]

sir while running the V8 benchmark tests, I encountered a STATUS_STACK_OVERFLOW when executing combined.js. To investigate, I cloned a fresh copy of boa-dev/boa (latest main, no modifications), built it in release mode, and reproduced the same crash.

When running with a larger stack (16 MB thread), all benchmarks completed successfully (Score: 214). also i ran some custom microbenchmarks around the specific patterns introduced by the optimisation passes, and that showed measurable performance improvements to the current boa

[jedel1043]

No worries! You're probably running this on Windows, which has a much smaller stack size by default than Linux. We're kinda working towards fixing that, but than you for testing it anyways!

[jedel1043]

Everything looks good! We can merge this after fixing all the CI warnings

[hansl]

Hey, are those test262 still failing? That is a huge red flag that something broke.