[llvm] [NVPTX] Improve lowering of v4i8 (PR #67866)

[Thomas Raoux via llvm-commits]

https://github.com/ThomasRaoux requested changes to this pull request.

Actually trying out the patch in triton it causes some invalid ptx to get emitted (see comment)

https://github.com/llvm/llvm-project/pull/67866