Do the right thing on NULL uint64 fields.

Patch by Clemens Hammacher!

Fixes PR12243

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@152880 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Analysis/DebugInfo.cpp b/lib/Analysis/DebugInfo.cpp
index 585a087..e30c0a9 100644
--- a/lib/Analysis/DebugInfo.cpp
+++ b/lib/Analysis/DebugInfo.cpp
@@ -68,7 +68,7 @@
     return 0;
 
   if (Elt < DbgNode->getNumOperands())
-    if (ConstantInt *CI = dyn_cast<ConstantInt>(DbgNode->getOperand(Elt)))
+    if (ConstantInt *CI = dyn_cast_or_null<ConstantInt>(DbgNode->getOperand(Elt)))
       return CI->getZExtValue();
 
   return 0;