[InstCombine] fix misnamed locals and use them to reduce code; NFCI

[InstCombine] fix misnamed locals and use them to reduce code; NFCI

We had these locals:
Value *Op0RHS = LHS->getOperand(1);
Value *Op1LHS = RHS->getOperand(0);
...so we confusingly transposed the meaning of left/right and op0/op1.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@312418 91177308-0d34-0410-b5e6-96231b3b80d8