DataFlowSanitizer: Remove unreachable BBs so IR continues to verify
under the args ABI.

Differential Revision: http://llvm-reviews.chandlerc.com/D1316

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@188113 91177308-0d34-0410-b5e6-96231b3b80d8