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 91.96429% with 27 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.28%. Comparing base (cb8a732) to head (85a9365).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
vyper/venom/check_venom.py	89.70%	6 Missing and 1 partial ⚠️
vyper/venom/basicblock.py	88.88%	4 Missing and 1 partial ⚠️
vyper/venom/passes/dft.py	75.00%	2 Missing and 2 partials ⚠️
vyper/venom/ir_node_to_venom.py	95.16%	2 Missing and 1 partial ⚠️
vyper/venom/function.py	0.00%	2 Missing ⚠️
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 ⚠️
vyper/venom/venom_to_assembly.py	95.65%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4747      +/-   ##
==========================================
- Coverage   93.30%   93.28%   -0.03%     
==========================================
  Files         137      137              
  Lines       19480    19623     +143     
  Branches     3367     3396      +29     
==========================================
+ Hits        18176    18305     +129     
- Misses        884      894      +10     
- Partials      420      424       +4     

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[charles-cooper]

this optimization is actually unnecessary right? will be handled by SCCP/algebraic opts