[llvm] [AMDGPU] Add intrinsic readanylane (PR #115696)

[Sameer Sahasrabuddhe via llvm-commits]

ssahasra wrote:

> I would say let's say it clearly that readanylane is defined only if the input is uniform when defined and at the use of readanylane call. We really want the intrinsic to be not convergent to allow related middle-end optimizations.

The change description starts with:

> "Sometimes, we know the value is uniform, but backend cannot easily prove that it is uniform."

Can this be solved with an attribute instead?

If we do need an intrinsic, we should stop comparing it with readfirstlane, and just focus on what readanylane means. Would it be correct to rename it to something like "assumeuniform"? Does this provide information that can be propagated backwards when initializing uniformity analysis?

https://github.com/llvm/llvm-project/pull/115696