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

2 files changed, 8 insertions, 5 deletions

diff --git a/include/llvm/Instructions.h b/include/llvm/Instructions.h
index fbf376a..d8ce835 100644
--- a/include/llvm/Instructions.h
+++ b/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) {
diff --git a/lib/VMCore/AsmWriter.cpp b/lib/VMCore/AsmWriter.cpp
index 6a17516..0322d66 100644
--- a/lib/VMCore/AsmWriter.cpp
+++ b/lib/VMCore/AsmWriter.cpp
@@ -1505,13 +1505,14 @@ void AssemblyWriter::printInstruction(const Instruction &I) {
   const Value *Operand = I.getNumOperands() ? I.getOperand(0) : 0;
 
   // Special case conditional branches to swizzle the condition out to the front
-  if (isa<BranchInst>(I) && I.getNumOperands() > 1) {
+  if (isa<BranchInst>(I) && cast<BranchInst>(I).isConditional()) {
+    BranchInst &BI(cast<BranchInst>(I));
     Out << ' ';
-    writeOperand(I.getOperand(2), true);
+    writeOperand(BI.getCondition(), true);
     Out << ", ";
-    writeOperand(Operand, true);
+    writeOperand(BI.getSuccessor(0), true);
     Out << ", ";
-    writeOperand(I.getOperand(1), true);
+    writeOperand(BI.getSuccessor(1), true);
 
   } else if (isa<SwitchInst>(I)) {
     // Special case switch statement to get formatting nice and correct...