dialects: (stablehlo) Add support for returnOp

[efferifick]

Add support for stablehlo.returnOp.

Note: this operation is not listed in the standard, but it is used by examples in the standard. See for example:

// %index: -1
// %result_branch0: [0, 0]
// %result_branch1: [1, 1]
%result0, %result1 = "stablehlo.case"(%index) ({
  "stablehlo.return"(%result_branch0, %result_branch0) : (tensor<2xi64>, tensor<2xi64>) -> ()
}, {
  "stablehlo.return"(%result_branch1, %result_branch1) : (tensor<2xi64>, tensor<2xi64>) -> ()
}) : (tensor<i32>) -> (tensor<2xi64>, tensor<2xi64>)
// %result0: [1, 1]
// %result1: [1, 1]

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.90%. Comparing base (04fcab7) to head (e80dd66).
Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3085      +/-   ##
==========================================
- Coverage   89.90%   89.90%   -0.01%     
==========================================
  Files         418      418              
  Lines       52914    52922       +8     
  Branches     8204     8205       +1     
==========================================
+ Hits        47571    47578       +7     
- Misses       4017     4018       +1     
  Partials     1326     1326              

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