fix[venom]: memmerge bytecodesize

[HodanPlodky]

What I did

Improvements in memmerge pass so the barriers dont need to flush all the copies. This was done in response to issue #4420

How I did it

How to verify it

Commit message

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Description for the changelog

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[codecov]

Codecov Report

Attention: Patch coverage is 87.23404% with 6 lines in your changes missing coverage. Please review.

Project coverage is 90.88%. Comparing base (4507d2a) to head (744a09a).

Files with missing lines	Patch %	Lines
vyper/venom/passes/memmerging.py	87.23%	2 Missing and 4 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4422      +/-   ##
==========================================
- Coverage   91.94%   90.88%   -1.06%     
==========================================
  Files         119      119              
  Lines       16677    16697      +20     
  Branches     2807     2814       +7     
==========================================
- Hits        15333    15175     -158     
- Misses        924     1059     +135     
- Partials      420      463      +43     

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