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

[Artem Belevich via llvm-commits]

Artem-B wrote:

We'll probably also need similar bitcast lowering for v2bf16. I suspect now that v2i16 are available we'll see LLVM picking them for various loads/stores and bf16 will likely end up using that path, too. Not sure if we'll need bitcasts between v2f16 and v2bf16 in practice, but it would not hurt to add them just in case, too.

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