Minor improve to assertion (#3295)

nnvm/src/core/graph.cc

@@ -107,7 +107,7 @@ IndexedGraph::IndexedGraph(const Graph &g) {
       for (const auto& nptr : n->control_deps) {
         if (!nptr->is_variable() && is_ghost.get(nptr->op(), false)) continue;
         auto it = node2index_.find(nptr.get());
-        CHECK(it != node2index_.end() && it->first == nptr.get());
+        CHECK(it != node2index_.end()) << "control dep not found in graph";
         control_deps_.push_back(it->second);
       }
       control_rptr.push_back(control_deps_.size());