feat[venom]: algebraic optimizations

[harkal]

What I did

Improve optimizations performed by algebraic optimization pass. Specifically, optimize add/sub chains.

How I did it

How to verify it

Commit message

Implements constant folding and chain optimization for add/sub operations in the Venom IR:
  - Adds `_fold_add_chain()` to collapse chains of add/sub operations into single operations
  - Adds `_trace_add_chain()` to trace through add/sub chains and compute total offsets
  - Adds `is_single_use()` helper to DFG analysis for checking variable usage
  - Optimize patterns like `add(add(x, c1), c2)` to `add(x, c1+c2)`
  - Handle sub operations in chains
  - Add foundation for shifted add chain optimization (currently disabled)

Description for the changelog

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[codecov]

Codecov Report

❌ Patch coverage is 68.62745% with 32 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.17%. Comparing base (cb8a732) to head (91a72c1).

Files with missing lines	Patch %	Lines
vyper/venom/passes/algebraic_optimization.py	68.00%	27 Missing and 5 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4778      +/-   ##
==========================================
- Coverage   93.30%   93.17%   -0.13%     
==========================================
  Files         137      137              
  Lines       19480    19580     +100     
  Branches     3367     3391      +24     
==========================================
+ Hits        18176    18244      +68     
- Misses        884      911      +27     
- Partials      420      425       +5     

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