R600/SI: Allow commuting some 3 op instructions

e.g. v_mad_f32 a, b, c -> v_mad_f32 b, a, c

This simplifies matching v_madmk_f32.

This looks somewhat surprising, but it appears to be
OK to do this. We can commute src0 and src1 in all
of these instructions, and that's all that appears
to matter.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221910 91177308-0d34-0410-b5e6-96231b3b80d8
R600/SI: Allow commuting with src2_modifiers

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221911 91177308-0d34-0410-b5e6-96231b3b80d8