[X86] As per suggestion from Craig Topper and Hal Finkel, override
TargetInstrInfo::findCommutedOpIndices to enable VFMA*231 commutation, rather
than abusing commuteInstruction.

Thanks very much for the suggestion guys!



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205489 91177308-0d34-0410-b5e6-96231b3b80d8