VMCore/BasicBlock.cpp: Don't assume BasicBlock::iterator might end with a...

VMCore/BasicBlock.cpp: Don't assume BasicBlock::iterator might end with a non-PHInode Instruction in successors.

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.

llvm-svn: 137175