feat[venom]: add multi-output instruction support for stack-based calling convention

[harkal]

What I did

Extent Venom to support multi-output instructions and implemented a multi-output invoke for functions that return multiple values.

How I did it

How to verify it

Commit message

Implements support for instructions with multiple return values, primarily for
the `invoke` instruction using stack-based return convention. Instructions can
now have multiple outputs via `_outputs` field, with `get_outputs()` and other
helpers.

Key changes:
  - Add `_outputs` field to IRInstruction for multi-output support
  - Fix parser, DFG analysis, and optimisation passes to handle multi-output
  - Update venom-to-assembly to properly manage stack for multi-return invoke
  - Add comprehensive tests for multi-output invoke instructions

Description for the changelog

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[codecov]

Codecov Report

❌ Patch coverage is 90.83095% with 32 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.23%. Comparing base (a68eb57) to head (fbd7108).

Files with missing lines	Patch %	Lines
vyper/venom/check_venom.py	88.31%	8 Missing and 1 partial ⚠️
vyper/venom/basicblock.py	89.18%	4 Missing ⚠️
vyper/venom/ir_node_to_venom.py	94.28%	3 Missing and 1 partial ⚠️
vyper/venom/passes/dft.py	63.63%	2 Missing and 2 partials ⚠️
vyper/venom/function.py	0.00%	3 Missing ⚠️
vyper/venom/venom_to_assembly.py	90.00%	1 Missing and 2 partials ⚠️
vyper/venom/parser.py	90.47%	1 Missing and 1 partial ⚠️
...r/venom/passes/common_subexpression_elimination.py	50.00%	1 Missing and 1 partial ⚠️
vyper/venom/analysis/dfg.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4747      +/-   ##
==========================================
- Coverage   93.27%   93.23%   -0.05%     
==========================================
  Files         137      137              
  Lines       19455    19616     +161     
  Branches     3361     3399      +38     
==========================================
+ Hits        18146    18288     +142     
- Misses        888      903      +15     
- Partials      421      425       +4     

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[charles-cooper]

check is not None

[charles-cooper]

nice work.

couple comments about the approach:

• why split the outputs into output + extra_outputs? seems like it would be cleaner to just have a list
• i think the use of truthy for outs is going to lead to a lot of trouble since it doesn't distinguish between None and the empty list -- ex. https://github.com/vyperlang/vyper/pull/4747/files#r2476313719

[HodanPlodky]

I suppose this is expected but just for sake of it not being forgotten, there is a small codesize regression in cases where the return values needs to be stored in memory after the call. Before this PR it could be done in the internal function it self but if the value is returned in stack then it needs to be done after every invoke. But I found at most 8 bytes of difference (in vault). Other than this note, it seems nice