perf: Cache dtype on ExprIR

[ritchie46]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 92.85714% with 4 lines in your changes missing coverage. Please review.

Project coverage is 79.50%. Comparing base (e03555c) to head (dc5a7b9).
Report is 38 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/polars-plan/src/plans/expr_ir.rs	93.87%	3 Missing ⚠️
...ates/polars-stream/src/physical_plan/lower_expr.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #20331   +/-   ##
=======================================
  Coverage   79.49%   79.50%           
=======================================
  Files        1569     1569           
  Lines      218634   218676   +42     
  Branches     2462     2462           
=======================================
+ Hits       173799   173848   +49     
+ Misses      44267    44260    -7     
  Partials      568      568           

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