[llvm] [AMDGPU] select v_sat_pk from two i16 or v2i16 (PR #121124)

[Pierre van Houtryve via llvm-commits]

https://github.com/Pierre-vh commented:

This looks fine to me but I will leave final approval to @arsenm because it's mostly codegen

https://github.com/llvm/llvm-project/pull/121124