[WIP] Add local value numbering implementation in ZJIT

[Copilot]

Implementation Plan for Local Value Numbering in ZJIT

Overview

Implement local value numbering (LVN) optimization to eliminate redundant computations within basic blocks. The Effect/AbstractHeap API already exists in the upstream codebase (zjit/src/hir_effect), so we'll use and potentially extend it as needed.

Implementation Checklist

• Rebase on upstream ruby/ruby master
• Explore existing Effect/AbstractHeap API
• Implement local_value_numbering() optimization pass
  • Create InsnKey type for hashing instructions by operation and operands
  • Track pure expressions within each basic block using HashMap
  • Use Union-Find (make_equal_to) to deduplicate redundant computations
  • Leverage existing has_effects() method to identify pure operations
• Add the pass to the optimize() pipeline
• Create comprehensive tests for LVN
  • Test deduplication of pure arithmetic operations
  • Test preservation of side-effecting operations
  • Test interaction with constant folding and DCE
• Run existing tests to ensure no regressions
• Build and validate implementation

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