[llvm] [NFC][TTI] Introduce getInstructionUniformity API for uniformity analysis (PR #168903)

[Jay Foad via llvm-commits]

jayfoad wrote:

You have replaced all generic uses of isSourceOfDivergence and isAlwaysUniform with the new hook, so you should remove those old functions from TTI. You can keep them as local helper methods in AMDGPUTargetTransformInfo and NVPTXTargetTransformInfo.

https://github.com/llvm/llvm-project/pull/168903