fix[venom]: fix MakeSSA with existing phis

[harkal]

What I did

Fixed the memssa pass to support already existing phi instructions.

How I did it

How to verify it

Commit message

This commit improves the `MakeSSA` pass to handle incoming `phi`
instructions. Following the implementation of the venom parser it
is possible to parse code with existing `phi` instructions in the
code. The old code was expecting the `phi` instructions to have
been self-placed and in "degenerate form" of output == all branch
arguments. The new code does not make this assumption.

This commit additionally:
- refactors the existing `make_ssa` tests to use new test machinery
- adds a phi parsing test (does not test the new code in this commit
  since it does not run passes, but does make sure we can at least parse
  phis)
- expands the venom round-trip tests to check that we can both
    a) run venom passes on parsed venom, and
    b) bytecode generation from round-tripping venom produced by the
       vyper frontend is equivalent to bytecode generation from the
       regular pipeline (directly from vyper source code)

Description for the changelog

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[codecov]

Codecov Report

Attention: Patch coverage is 10.00000% with 9 lines in your changes missing coverage. Please review.

Project coverage is 51.79%. Comparing base (9c98b3e) to head (52591ed).

Files with missing lines	Patch %	Lines
vyper/venom/basicblock.py	14.28%	6 Missing ⚠️
vyper/venom/passes/make_ssa.py	0.00%	3 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (9c98b3e) and HEAD (52591ed). Click for more details.

HEAD has 113 uploads less than BASE

Flag	BASE (9c98b3e)	HEAD (52591ed)
	140	27

Additional details and impacted files

@@             Coverage Diff             @@
##           master    #4423       +/-   ##
===========================================
- Coverage   91.76%   51.79%   -39.97%     
===========================================
  Files         119      119               
  Lines       16615    16615               
  Branches     2796     2796               
===========================================
- Hits        15247     8606     -6641     
- Misses        936     7364     +6428     
- Partials      432      645      +213     

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[charles-cooper]

i added more tests (1728bee runs venom passes in the round-trip tests) and there still seems to be some bugs, e.g. https://github.com/vyperlang/vyper/actions/runs/12483208026/job/34838622968?pr=4423 throws things like

at first glance, it looks like the fix fails if there are multiple phis in multiple basic blocks referencing the same variable name, but not sure