[DAGCombiner] Updated bswap byte offset variable names to be more descriptive. NFC

As discussed on D32039, use MaskByteOffset to describe the variable and also pull out repeated getOpcode() calls.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@301193 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed