[llvm] [IR] Add `alwaysuniform` function attribute (PR #214644)

[Justin Fargnoli via llvm-commits]

justinfargnoli wrote:

>>We should have an in-tree use case to motivate this.
>
> Not quite sure what you're asking for -- you want some in-tree frontend to emit this attribute? I don't think that is usually a requirement for new IR features. We have IR tests that show how it can be used and what effect it has.

Based on the description, I assumed `alwaysuniform` would primarily be used by intrinsics rather than emitted by frontends. If that's correct, then I think it makes sense for an in-tree intrinsic to use this to demonstrate its value.

I'm happy to kick this up to @nikic @arsenm. I may just be misunderstanding LLVM's conventions here. 

https://github.com/llvm/llvm-project/pull/214644