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

On sm_90 some instructions now support i16x2 which allows hardware to
execute more efficiently add, min and max instructions.

In order to support that we need to make i16x2 a native type in the
backend. This does the necessary changes to make i16x2 a native type and
adds support for the instructions natively supporting i16x2.

This caused a negative test in nvptx slp to start passing. Changed the
test to a positive one as the IR is correctly vectorized.