[PATCH] Fix crash in CFGReachabilityAnalysis triggered by IdempotentOperationChecker.

[Alexander Kornienko]

  I've added info on reproduction.

  The relevant stack frames are:
  clang::CFGReverseBlockReachabilityAnalysis::isReachable CFGReachabilityAnalysis.cpp:37
  (anonymous namespace)::IdempotentOperationChecker::pathWasCompletelyAnalyzed IdempotentOperationChecker.cpp:559
  (anonymous namespace)::IdempotentOperationChecker::checkEndAnalysis IdempotentOperationChecker.cpp:378

http://llvm-reviews.chandlerc.com/D2427