[llvm] [AMDGPU] Support Wave Reduction for i16 types - 1 (PR #194808)

[Juan Manuel Martinez Caamaño via llvm-commits]

https://github.com/jmmartinez commented:

I'm not confident with my knowledge of the regbank select part (it look reasonable to me though).

The rest looks good to me. Left 2 minor remarks.

https://github.com/llvm/llvm-project/pull/194808