[TargetInstrInfo] Fix the implementation of commuteInstruction to match the
comment of the API.

Relaxes the behavior of TargetInstrInfo::commuteInstruction when
TargetInstrInfo::findCommutedOpIndices returns false.

Previously TargetInstrInfo triggered a fatal error in such situation whereas based
on the comment in the API it should just return nullptr. Indeed the only
precondition that should be ensured is that the instruction must be commutable.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@208371 91177308-0d34-0410-b5e6-96231b3b80d8