AMDGPU/GlobalISel: Legalize G_FMAD

Unlike SelectionDAG, treat this as a normally legalizable operation.
In SelectionDAG this is supposed to only ever formed if it's legal,
but I've found that to be restricting. For AMDGPU this is contextually
legal depending on whether denormal flushing is allowed in the use
function.

Technically we currently treat the denormal mode as a subtarget
feature, so custom lowering could be avoided. However I consider this
to be a defect, and this should be contextually dependent on the
controllable rounding mode of the parent function.

llvm-svn: 371800