[DAGCombiner] add helper function for visitORLike; NFCI

[DAGCombiner] add helper function for visitORLike; NFCI

This combines all of the equivalent clean-ups for foldAndOfSetCCs:
https://reviews.llvm.org/rL298938
https://reviews.llvm.org/rL298940
https://reviews.llvm.org/rL298944
https://reviews.llvm.org/rL298949
https://reviews.llvm.org/rL298950
https://reviews.llvm.org/rL299002
https://reviews.llvm.org/rL299013

The sins of code duplication are on full display here:
each function is missing a fold that wasn't copied over from its logical sibling.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@299091 91177308-0d34-0410-b5e6-96231b3b80d8