DataFlowSanitizer: fix a use-after-free.  Spotted by libgmalloc.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@188216 91177308-0d34-0410-b5e6-96231b3b80d8
Reapply r188119 now that the bug it exposed is fixed.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@188217 91177308-0d34-0410-b5e6-96231b3b80d8