[DAGCombiner] Fix invalid alias analysis.

Summary:
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
code review.

Fixes PR41855.

Reviewers: niravd, gchatelet, EricWF

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D61846

llvm-svn: 360566