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

[Artem Belevich via llvm-commits]

Artem-B wrote:

Hmm. Why did they work then? Loads/stores/bitcasts for v2i16 should not have dependent on whether the target is sm_90 or not. Or maybe it did, if we set lowering action to Expand on older GPUs. 

You may try running the tests with `-mcpu=sm_90`

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