Revert "[AMDGPU] Select v_sat_pk_u8_i16"

This reverts commit 64b45db3.

Reason: the patterns are wrong which can result in a miscompilation.
However, fixing the pattern is not trivial due to how i8 values
are handled, and due to the additional type-checking performed by
D147127: trunc/smax/smin are all defined as int ops in the DAG
despite them working on vectors too.

As this is not a much-needed pattern, I prefer reverting for now
until I can find time to properly rewrite the pattern.