[X86] Remove the places that return nullptr from X86InstrInfo::commuteInstructionImpl.

findCommutedOpIndices does the pre-checking for whether commuting is possible. There should be no reason left to fail in commuteInstructionImpl. There was a missing pre-check that I've added there and changed the check to an assert in commuteInstructionImpl.

llvm-svn: 336070