perf[venom]: improve performance of MakeSSA

[charles-cooper]

What I did

How I did it

How to verify it

Commit message

`IRVariable` construction in `MakeSSA` is a hotspot. remove
constructions of `IRVariable`s when possible.

Description for the changelog

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.07%. Comparing base (8a93dd2) to head (a4dfdce).
Report is 97 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4491   +/-   ##
=======================================
  Coverage   92.07%   92.07%           
=======================================
  Files         120      120           
  Lines       17343    17352    +9     
  Branches     2937     2938    +1     
=======================================
+ Hits        15968    15977    +9     
  Misses        957      957           
  Partials      418      418           

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