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

[Jay Foad via llvm-commits]

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jayfoad wrote:

> I would strongly prefer removing the existing hooks as part of this patch.

How about doing it as an NFC refactoring _before_ this patch?

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