[llvm] [AMDGPU][TTI] Add Target Hook for Instruction Uniformity (getInstructionUniformity) (PR #137639)

[Matt Arsenault via llvm-commits]

https://github.com/arsenm commented:

I don't know what this new hook gives that the existing ones do not handle already 

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