dialects: (scf) Add parse/print to scf.if

[alexarice]

Adds custom parsing to some scf ops and fixes the relevant tests.

I noticed during this that scf.reduce is now quite different to upstream mlir, but didn't attempt to change this.

[alexarice]

The implementation of reduce seems to disagree with the fixed mlir commit too, so will likely have to revert that for now

[superlopuh]

jsut scf.if would already be great as a standalone PR, will also fix #2544

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.92%. Comparing base (30dcaa1) to head (79accaa).
Report is 10 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3126   +/-   ##
=======================================
  Coverage   89.91%   89.92%           
=======================================
  Files         425      425           
  Lines       53391    53446   +55     
  Branches     8268     8278   +10     
=======================================
+ Hits        48006    48060   +54     
+ Misses       4047     4045    -2     
- Partials     1338     1341    +3     

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