[CodeGen] Make the TwoAddressInstructionPass check if the instruction is commutable...

[CodeGen] Make the TwoAddressInstructionPass check if the instruction is commutable before calling findCommutedOpIndices for every operand. Also make sure the operand is a register before each call to save some work on commutable instructions that might have an operand.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@281158 91177308-0d34-0410-b5e6-96231b3b80d8