[AMDGPU] Select v_sat_pk_u8_i16

The backend knew about `v_sat_pk_u8_i16` but never made use of it.
This patch adds selection patterns (DAG/GISel) for that instruction.
I think it'll be very rarely used, but at least it's possible to use it.

Solves #58266 (https://github.com/llvm/llvm-project/issues/58266)

Reviewed By: foad

Differential Revision: https://reviews.llvm.org/D144729