[DAGCombiner] Fix invalid alias analysis.
When we know for sure whether two addresses do or do not alias, we
should immediately return from DAGCombiner::isAlias().
I think this comes from a bad copy/paste, Sorry for not catching that during the
Reviewers: niravd, gchatelet, EricWF
Subscribers: hiraditya, llvm-commits
Differential Revision: https://reviews.llvm.org/D61846