refactor: Refactor parts of IR.

[ritchie46]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 92.10526% with 3 lines in your changes missing coverage. Please review.

Project coverage is 81.42%. Comparing base (2dba716) to head (3c3fb17).

Files	Patch %	Lines
...lars-plan/src/logical_plan/conversion/dsl_to_ir.rs	50.00%	1 Missing ⚠️
...tes/polars-plan/src/logical_plan/conversion/mod.rs	0.00%	1 Missing ⚠️
crates/polars-plan/src/logical_plan/ir/dot.rs	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16899      +/-   ##
==========================================
- Coverage   81.43%   81.42%   -0.02%     
==========================================
  Files        1425     1425              
  Lines      187987   187970      -17     
  Branches     2704     2704              
==========================================
- Hits       153093   153056      -37     
- Misses      34397    34417      +20     
  Partials      497      497              

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