[AMDGPU][GlobalISel] Support mad/fma_mix selection

Adds support for selecting the following instructions using GlobalISel:
- v_mad_mix/v_fma_mix
- v_mad_mixhi/v_fma_mixhi
- v_mad_mixlo/v_fma_mixlo

To select those instructions properly, some additional changes were
needed which impacted other tests as well.

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D134354