[AMDGPU] Use `v_cvt_pk_*` instructions for i16_f32 saturated conversions (#202680)
This is a follow-up to #187487. `v_cvt_pk_*` is used for vector cases, as well as for scalar types (by passing a dummy second input) on GFX11+. Relevant fallback patterns have also been added and `splitUnaryVectorOp` has been extended to handle trailing scalar ops if present.
Assisted-by: Claude Code