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

[Jay Foad via llvm-commits]

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jayfoad wrote:

If you're adding this new hook, you should aim to remove the existing hooks `isSourceOfDivergence` and `isAlwaysUniform`, since the new one is strictly more capable. This could be done as a separate NFC refactoring.

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