[NFC][DAGCombine] unfoldMaskedMerge(): rename two variables

The current names can be confused with the A and B sides
of the canonical masked merge pattern.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@331609 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed