fix: Enable CSE in eager if struct are expanded

[ritchie46]

This will ensure you get stable output as expected. Fixes #18411

[codecov]

Codecov Report

Attention: Patch coverage is 95.45455% with 3 lines in your changes missing coverage. Please review.

Project coverage is 79.94%. Comparing base (20ce0f1) to head (f543d18).
Report is 5 commits behind head on main.

Files	Patch %	Lines
...ates/polars-plan/src/plans/conversion/dsl_to_ir.rs	90.90%	2 Missing ⚠️
...polars-plan/src/plans/conversion/expr_expansion.rs	97.14%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #18426      +/-   ##
==========================================
+ Coverage   79.93%   79.94%   +0.01%     
==========================================
  Files        1495     1496       +1     
  Lines      200277   200190      -87     
  Branches     2867     2867              
==========================================
- Hits       160082   160051      -31     
+ Misses      39649    39593      -56     
  Partials      546      546              

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[ritchie46]

CI failure is unrelated.