[llvm] [AMDGPU] Add getCmpSelInstrCost override to re-enable SimplifyCFG speculation for vector types (PR #208043)

[Matt Arsenault via llvm-commits]

https://github.com/arsenm requested changes to this pull request.

The added CostModel test does not cover the change. It is adding independent compare and select tests (which I would hope are already covered by existing tests, if not there's should be one test per instruction type).

The cost model test for this case needs to include the compare used by the select 

https://github.com/llvm/llvm-project/pull/208043