[AMDGPU] Add patterns for mad/mac legacy f32 instructions

Note that all subtargets up to GFX10.1 have v_mad_legacy_f32, but GFX8/9
lack v_mac_legacy_f32. GFX10.3 has no mad/mac f32 instructions at all.

Differential Revision: https://reviews.llvm.org/D88890