refactor[venom]: refactor venom operand classes

[harkal]

What I did

How I did it

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Commit message

the original `IRValue`, `IRVariable`, `IRLabel` and `IROperand`
hierarchy got convoluted after several refactors. this commit refactors
the hierarchy so that the relationship between the classes is clearer.

Description for the changelog

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[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.87%. Comparing base (7485cea) to head (82a423a).

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3915      +/-   ##
==========================================
- Coverage   90.92%   90.87%   -0.05%     
==========================================
  Files          95       95              
  Lines       14411    14395      -16     
  Branches     3189     3187       -2     
==========================================
- Hits        13103    13082      -21     
- Misses        908      912       +4     
- Partials      400      401       +1     

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[charles-cooper]

seems fine. i think we should have explicit __eq__() implementations, and as a style thing i personally prefer explicit isinstance checks (friendlier to mypy) instead of the utility functions, but it is OK either way tbh.

[harkal]

seems fine. i think we should have explicit __eq__() implementations, and as a style thing i personally prefer explicit isinstance checks (friendlier to mypy) instead of the utility functions, but it is OK either way tbh.

I reverted the utility functions, isinstance is the way everywhere now. Code check complaining about __eq__() is immaterial, the methods will be exactly the same in all subclasses.