Frontends(eg. clang) might pass incomplete form of IR, to step off the way beyond iterator end. In the case I had met, it took infinite loop due to meeting bogus PHInode.
Thanks to Jay Foad and John McCall.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@137175 91177308-0d34-0410-b5e6-96231b3b80d8