!7306 Fix checking nop for a cnode whose first input is not a value node

Merge pull request !7306 from YuJianfeng/master

mindspore/ccsrc/backend/optimizer/common/helper.cc

@@ -405,6 +405,14 @@ bool IsNopNode(const AnfNodePtr &node) {
   }
   CNodePtr cnode = node->cast<CNodePtr>();
   MS_EXCEPTION_IF_NULL(cnode);
+  if (cnode->inputs().empty()) {
+    return false;
+  }
+  auto input0 = cnode->input(0);
+  MS_EXCEPTION_IF_NULL(input0);
+  if (!input0->isa<ValueNode>()) {
+    return false;
+  }
   bool is_nop_node = false;
   if (AnfAlgo::HasNodeAttr("nop_op", cnode)) {
     is_nop_node = AnfAlgo::GetNodeAttr<bool>(cnode, "nop_op");