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

[Artem Belevich via llvm-commits]

Artem-B wrote:

Looks like we've missed lowering of bitcasts between v2f16 and v2i16 and it breaks XLA.

```
LLVM ERROR: Cannot select: t119: v2f16 = bitcast t118
  t118: v2i16 = or t375, t401
    t375: v2i16 = BUILD_VECTOR t374, t372
      t374: i16 = select t247, Constant:i16<8960>, t360
```


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