AMDGPU/GlobalISel: Add integer med3 combines

Add signed and unsigned integer version of med3 combine.
Source pattern is min(max(Val, K0), K1) or max(min(Val, K1), K0)
where K0 and K1 are constants and K0 <= K1. Destination is med3
that corresponds to signedness of min/max in source.

Differential Revision: https://reviews.llvm.org/D90050