[AMDGPU] Simplify mad/mac patterns. NFC.

Simplify instruction selection patterns for mad/mac:
- Use any_fmad consistently to make it clear that all patterns treat
  fmad and AMDGPUfmad_ftz identically.
- For mad, put the patterns on the instruction definitions. For mac the
  patterns are still out-of-line because we want to set AddedComplexity
  and to have special handling of the source modifiers.

Differential Revision: https://reviews.llvm.org/D133305