transformations: CSE: Refactor for reuse-friendliness

[PapyChacal]

Refactor CSEDrive to make most things private and mostly expose simplify, applying CSE on one Operation/Block/Region.
Then just expose a public cse function as entry-point: any rewrite can now just import cse and use it during other transformation 🙂

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.58%. Comparing base (20707cb) to head (6a3d660).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2642      +/-   ##
==========================================
- Coverage   89.58%   89.58%   -0.01%     
==========================================
  Files         360      360              
  Lines       46005    46020      +15     
  Branches     6964     6966       +2     
==========================================
+ Hits        41215    41225      +10     
- Misses       3718     3722       +4     
- Partials     1072     1073       +1     

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