make sure that BranchInst::getSuccessor() does not assert in cast<>

even if the underlying operand is NULL. This may happen in debugging context
within opt with partial loop unrolling (see test/Transforms/LoopUnroll/partial.ll).
After this fix I can resubmit the (backed out) r63459:
* lib/VMCore/AsmWriter.cpp: use precise accessors.

llvm-svn: 64142