While replacing an MDNode elment, properly update MDNode's operand list. 
MDNode's operand list does not include all elements.

llvm-svn: 81045
diff --git a/llvm/lib/VMCore/LLVMContext.cpp b/llvm/lib/VMCore/LLVMContext.cpp
index 887679e..7d233d9 100644
--- a/llvm/lib/VMCore/LLVMContext.cpp
+++ b/llvm/lib/VMCore/LLVMContext.cpp
@@ -62,9 +62,10 @@
       return Changed;
 
     while (!DeadMDNodes.empty()) {
-      const MDNode *N = cast<MDNode>(DeadMDNodes.back()); DeadMDNodes.pop_back();
-      if (N->use_empty())
-	delete N;
+      Value *V = DeadMDNodes.back(); DeadMDNodes.pop_back();
+      if (const MDNode *N = dyn_cast_or_null<MDNode>(V))
+        if (N->use_empty())
+          delete N;
     }
   }
   return Changed;