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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@64142 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/Instructions.h

@@ -2186,7 +2186,9 @@ public:
 
   BasicBlock *getSuccessor(unsigned i) const {
     assert(i < getNumSuccessors() && "Successor # out of range for Branch!");
-    return cast<BasicBlock>(getOperand(i));
+    if (Value *V = getOperand(i))
+      return cast<BasicBlock>(V);
+    return 0;
   }
 
   void setSuccessor(unsigned idx, BasicBlock *NewSucc) {