[llvm] [AMDGPU] Enable vectorization of i8 values. (PR #134934)

[Jeffrey Byrnes via llvm-commits]

https://github.com/jrbyrnes commented:

LGTM but please wait for @arsenm for approval.

The one outstanding concern that we discussed offline (I think) is that this method of vectorizing does not encourage vectorization of other illegal types that may benefit from cross BB type coercion. We don't really have any strong present need that I know of, but we may encounter in the future. But, it is probably best to address such a need as it occurs rather than spending a bunch of time trying to future proof.

https://github.com/llvm/llvm-project/pull/134934