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

[Matt Arsenault via llvm-commits]

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arsenm wrote:

I don't understand the fixme, or falling back on the base implementation. The true implementation should be straightforward?

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