dialects: (stablehlo) add stablehlo.case

[efferifick]

Adds stablehlo.case

[efferifick]

Noticed that multiple returns are printed differently in xdsl.

// CHECK: %0, %1 = "stablehlo.case"(%index) ({
%0:2 = "stablehlo.case"(%index) ({

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.89%. Comparing base (1e488d7) to head (ae5cad8).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3095   +/-   ##
=======================================
  Coverage   89.89%   89.89%           
=======================================
  Files         419      419           
  Lines       53017    53027   +10     
  Branches     8221     8222    +1     
=======================================
+ Hits        47660    47671   +11     
  Misses       4028     4028           
+ Partials     1329     1328    -1     

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[superlopuh]

Looks good modulo suggested changes

[efferifick]

@superlopuh fixed merge conflicts