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

[Artem Belevich via llvm-commits]

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Artem-B wrote:

What's the story with these test files?  Are they relevant to v2i16 changes in the rest of the patch? If not, they should go into a separate patch.

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