Automerge: [SandboxVec][DAG] Fix interval check without Node

This patch moves the check of whether a node exists before the check of
whether it is contained in the interval.

Author: Vasileios Porpodas

llvm/lib/Transforms/Vectorize/SandboxVectorizer/DependencyGraph.cpp

@@ -267,11 +267,11 @@ void DependencyGraph::setDefUseUnscheduledSuccs(
       auto *OpI = dyn_cast<Instruction>(Op);
       if (OpI == nullptr)
         continue;
-      if (!TopInterval.contains(OpI))
-        continue;
       auto *OpN = getNode(OpI);
       if (OpN == nullptr)
         continue;
+      if (!TopInterval.contains(OpI))
+        continue;
       ++OpN->UnscheduledSuccs;
     }
   }