fix: optimizer regression

[charles-cooper]

What I did

fix overly aggressive optimizations introduced in #2647

How I did it

roll back the optimization if a side-effecting operation is not found in the optimized result. the heuristic is overly conservative, which i am ok with.

How to verify it

see code, tests

Commit message

Description for the changelog

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[codecov-commenter]

Codecov Report

Merging #2868 (f3a8237) into master (3195701) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2868      +/-   ##
==========================================
+ Coverage   87.67%   87.70%   +0.02%     
==========================================
  Files          94       94              
  Lines       10183    10214      +31     
  Branches     2501     2509       +8     
==========================================
+ Hits         8928     8958      +30     
  Misses        790      790              
- Partials      465      466       +1     

Impacted Files	Coverage Δ
vyper/codegen/ir_node.py	92.80% <100.00%> (ø)
vyper/ir/optimizer.py	91.72% <100.00%> (+0.32%)	⬆️
vyper/codegen/external_call.py	97.24% <0.00%> (-0.83%)	⬇️
vyper/compiler/output.py	93.78% <0.00%> (ø)
vyper/ir/compile_ir.py	93.69% <0.00%> (+0.06%)	⬆️
vyper/semantics/validation/local.py	91.13% <0.00%> (+0.27%)	⬆️

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[charles-cooper]

The reason I am leaving this as a draft is because if we really want to fix the optimizer here, we need to get a bit more principled. Basically, we need an effects graph and machinery to ensure we don't break the effects graph. So, if we have other priorities besides the optimizer, an alternative is to rollback #2647 and leave this optimizer project for later.