Give 'hasPath' a longer but clearer name 'isPotentiallyReachable'. Also expand
the comment. No functionality change. This change broken out of
http://llvm-reviews.chandlerc.com/D996 .


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@186558 91177308-0d34-0410-b5e6-96231b3b80d8