feat[venom]: make cfg scheduler "stack aware"

[charles-cooper]

What I did

How I did it

How to verify it

Commit message

this is a step towards making the cfg traversal order more "stack
aware". previously we would blindly try to remove `iszero` instructions
before a `jnz`. now, the heuristic is based on which basic block
has a smaller sized dependency on this basic block. only if the two
cfg_out blocks have the same sized dependency, then we try to remove
the iszero.

this creates a slight improvement to codesize due to fewer stack
operations at `jnz` boundaries.

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[codecov]

Codecov Report

Attention: Patch coverage is 11.11111% with 16 lines in your changes missing coverage. Please review.

Project coverage is 45.99%. Comparing base (471a556) to head (22fd559).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
vyper/venom/passes/branch_optimization.py	11.11%	16 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (471a556) and HEAD (22fd559). Click for more details.

HEAD has 138 uploads less than BASE

Flag	BASE (471a556)	HEAD (22fd559)
	139	1

Additional details and impacted files

@@             Coverage Diff             @@
##           master    #4356       +/-   ##
===========================================
- Coverage   91.37%   45.99%   -45.39%     
===========================================
  Files         112      112               
  Lines       15927    15940       +13     
  Branches     2693     2694        +1     
===========================================
- Hits        14554     7331     -7223     
- Misses        939     8050     +7111     
- Partials      434      559      +125     

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