refactor[venom]: add new venom test machinery

[charles-cooper]

What I did

refactor venom tests with new parsing machinery. it should make tests easier to read. add an example of the new testing style

How I did it

How to verify it

Commit message

refactor venom tests with new parsing machinery. it should make tests
easier to read. refactor `test_simplify_cfg.py` as an example of the
new testing style

additionally add some parser / text format fixes:

- switch order of operands in `phi` text format
- fix `jnz` parsing to match `IRInstruction.__repr__()`
- add comments to venom grammar (`;` indicates a comment a la llvm)
- make data section optional
- fix grammar rule: expr can be any operand.

Description for the changelog

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[codecov]

Codecov Report

Attention: Patch coverage is 0% with 7 lines in your changes missing coverage. Please review.

Project coverage is 50.92%. Comparing base (537313b) to head (b12f75e).

Files with missing lines	Patch %	Lines
vyper/venom/parser.py	0.00%	6 Missing ⚠️
vyper/venom/basicblock.py	0.00%	1 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (537313b) and HEAD (b12f75e). Click for more details.

HEAD has 183 uploads less than BASE

Flag	BASE (537313b)	HEAD (b12f75e)
	200	17

Additional details and impacted files

@@             Coverage Diff             @@
##           master    #4401       +/-   ##
===========================================
- Coverage   91.11%   50.92%   -40.20%     
===========================================
  Files         115      115               
  Lines       16225    16227        +2     
  Branches     2728     2729        +1     
===========================================
- Hits        14784     8264     -6520     
- Misses       1006     7345     +6339     
- Partials      435      618      +183     

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[HodanPlodky]

I now this is not necessarily part of the PR but there it should be self.opcode no? since the opcode contains space after original opcode so body of this if is unreachable right now (I have noticed it when thinking about order of operands in jnz)

[charles-cooper]

yea seems so. let's leave it out of scope, can be fixed in #4402 or #4403