[llvm] [AMDGPU][TTI] Add target hook for the custom instruction uniformity (PR #137639)

[Sameer Sahasrabuddhe via llvm-commits]

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ssahasra wrote:

I would strongly prefer removing the existing hooks as part of this patch. It just might expose rough corners that we need to consider in this design itself.

https://github.com/llvm/llvm-project/pull/137639