refactor[venom]: strengthen an assumption in venom lowering

[charles-cooper]

this commit strengthens an assumption in venom_to_assembly. due to SSA form, the arguments to phi must be in the predecessor blocks. this means that the def does not reach any use from both branches, except in the case that the use is a phi.

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[codecov]

Codecov Report

❌ Patch coverage is 86.04651% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.04%. Comparing base (b74f825) to head (a6352be).
⚠️ Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
vyper/venom/check_venom.py	25.00%	3 Missing ⚠️
vyper/venom/passes/simplify_cfg.py	25.00%	1 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4715      +/-   ##
==========================================
+ Coverage   93.00%   93.04%   +0.03%     
==========================================
  Files         131      131              
  Lines       19095    19167      +72     
  Branches     3324     3329       +5     
==========================================
+ Hits        17760    17834      +74     
  Misses        898      898              
+ Partials      437      435       -2     

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