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

[Jay Foad via llvm-commits]

jayfoad wrote:

> Don't we already have special context dependent uniformity with the existing API? e.g. identifying math based on the work item ID

Yes we have some special known-uniform cases in GCNTTIImpl::isAlwaysUniform, but that code does not have access to the uniformity of the operands, so it cannot implement rules like "result is uniform if any operand is uniform".

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