[DAG] move fold (select C, 0, 1 -> xor C, 1) to a helper function; NFC

We're missing at least 3 other similar folds based on what we have in InstCombine. 


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@283596 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed