Depsgraph: Introduce operation code for visibility operation

No functional changes, just makes code more semantically clear.

source/blender/depsgraph/intern/builder/deg_builder_nodes.cc

@@ -180,7 +180,7 @@ IDNode *DepsgraphNodeBuilder::add_id_node(ID *id)
 
     ComponentNode *visibility_component = id_node->add_component(NodeType::VISIBILITY);
     OperationNode *visibility_operation = visibility_component->add_operation(
-        nullptr, OperationCode::OPERATION);
+        nullptr, OperationCode::VISIBILITY);
     /* Pin the node so that it and its relations are preserved by the unused nodes/relations
      * deletion. This is mainly to make it easier to debug visibility. */
     visibility_operation->flag |= OperationFlag::DEPSOP_FLAG_PINNED;

source/blender/depsgraph/intern/node/deg_node_operation.cc

@@ -32,6 +32,8 @@ const char *operationCodeAsString(OperationCode opcode)
       return "PARAMETERS_EVAL";
     case OperationCode::PARAMETERS_EXIT:
       return "PARAMETERS_EXIT";
+    case OperationCode::VISIBILITY:
+      return "VISIBILITY";
     /* Animation, Drivers, etc. */
     case OperationCode::ANIMATION_ENTRY:
       return "ANIMATION_ENTRY";

source/blender/depsgraph/intern/node/deg_node_operation.h

@@ -34,6 +34,7 @@ enum class OperationCode {
   PARAMETERS_ENTRY,
   PARAMETERS_EVAL,
   PARAMETERS_EXIT,
+  VISIBILITY,
 
   /* Animation, Drivers, etc. --------------------------------------------- */
   /* NLA + Action */