[AMDGPU] Convert mac/fmac to mad/fma when folding output modifiers

Use of output modifiers forces VOP3 encoding for a VOP2 mac/fmac
instruction, so we might as well convert it to the more flexible VOP3-
only mad/fma form.

With this change, the only way we should emit VOP3-encoded mac/fmac is
if regalloc chooses registers that require the VOP3 encoding, e.g. sgprs
for both src0 and src1. In all other cases the mac/fmac should either be
converted to mad/fma or shrunk to VOP2 encoding.

Differential Revision: https://reviews.llvm.org/D110156