Fix IROperator::is_const's handling of casts.

[alexreinking]

The IROperator::is_const function didn't check can_represent before stripping a cast. That allowed it to confuse a 255 for a -1.

Fixes #9242

Checklist

• Tests added or updated (not required for docs, CI config, or typo fixes)
• Commits include AI attribution where applicable (see Code of Conduct)

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 70.28%. Comparing base (bb9a42a) to head (7d55cea).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/IROperator.cpp	50.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9243      +/-   ##
==========================================
- Coverage   70.30%   70.28%   -0.02%     
==========================================
  Files         255      255              
  Lines       78904    78949      +45     
  Branches    18866    18889      +23     
==========================================
+ Hits        55470    55491      +21     
+ Misses      17849    17846       -3     
- Partials     5585     5612      +27     

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