[llvm] [NVPTX] Make i16x2 a native type and add supported vec instructions (PR #65432)

[Artem Belevich via llvm-commits]

Artem-B wrote:

In any case, it probably does not matter much one way or another -- it's just one more cherry-picked revision to include into a new pull request.

https://github.com/llvm/llvm-project/pull/65432