refactor[venom]: refactor mem2var

[charles-cooper]

What I did

How I did it

How to verify it

Commit message

remove special cases which were necessary before introduction of
`palloca`. now they represent useless variable read/writes which are
safe to remove. results in removal of a few instructions on benchmark
contracts.

Description for the changelog

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[codecov]

Codecov Report

Attention: Patch coverage is 90.47619% with 2 lines in your changes missing coverage. Please review.

Project coverage is 90.80%. Comparing base (c9d8f5b) to head (5129d3d).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
vyper/venom/passes/mem2var.py	90.47%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4421      +/-   ##
==========================================
- Coverage   91.78%   90.80%   -0.99%     
==========================================
  Files         119      119              
  Lines       16615    16615              
  Branches     2796     2794       -2     
==========================================
- Hits        15250    15087     -163     
- Misses        935     1063     +128     
- Partials      430      465      +35     

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