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

[via llvm-commits]

ThomasRaoux wrote:

looking at it we do have some tests for bitcast 2xhalf -> 2xi16:
https://github.com/llvm/llvm-project/blob/f2b04434fc49e41444d1a1a4065e8fbe249538c2/llvm/test/CodeGen/NVPTX/f16x2-instructions.ll#L998

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