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

[Artem Belevich via llvm-commits]

Artem-B wrote:

I still need to test it on live code, though we do not have much code that would end up using v4i8.

The generated PTX checked in llvm/test/CodeGen/NVPTX/i8x4-instructions.ll could use extra scrutiny.

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