[InstCombine] rename variable for readability; NFC
[InstCombine] rename variable for readability; NFC

There's more that can be done here, but "OpI"
doesn't convey that we casted to BinaryOperator.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@371682 91177308-0d34-0410-b5e6-96231b3b80d8

GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm