[DFSan] Change the way labels are propagated when comparing memory through libc functions.
    
Differential Revision: http://llvm-reviews.chandlerc.com/D2252


git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@196388 91177308-0d34-0410-b5e6-96231b3b80d8