Add tests for the various combines. This should
always be at least cycle neutral on all subtargets for f64,
and faster on some. For f32 we should prefer selecting
v_mad_f32 over v_fma_f32.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@227484 91177308-0d34-0410-b5e6-96231b3b80d8
