**Apache Airflow version**: 2.1.0

**What happened**:

Using a Label infront of a TaskGroup breaks task dependencies, as tasks before the TaskGroup will now point to the last task in the TaskGroup.

**What you expected to happen**:

Task dependencies with or without Labels should be the same (and preferably the Label should be visible outside of the TaskGroup).
Behavior without a Label is as follows:

```python
with TaskGroup("tg1", tooltip="Tasks related to task group") as tg1:
        DummyOperator(task_id="b") >> DummyOperator(task_id="c")

DummyOperator(task_id="a") >> tg1
```
![without Taskgroup](https://user-images.githubusercontent.com/35238779/128502690-58b31577-91d7-4b5c-b348-4779c9286958.png)

**How to reproduce it**:

```python
with TaskGroup("tg1", tooltip="Tasks related to task group") as tg1:
        DummyOperator(task_id="b") >> DummyOperator(task_id="c")

    DummyOperator(task_id="a") >> Label("testlabel") >> tg1
```
![with TaskGroup](https://user-images.githubusercontent.com/35238779/128502758-5252b7e7-7c30-424a-ac3a-3f9a99b0a805.png)


#
            
            
            
            acyclic = False
            for node in list(graph_unsorted.values()):
                for edge in node.upstream_list:
                    if edge.node_id in graph_unsorted:
                        break
                    
                    tg = edge.task_group
                    while tg:
                        if tg.node_id in graph_unsorted:
                            break
                        tg = tg.task_group
                    if tg:
                        
                        break
                else:
                    acyclic = True
                    del graph_unsorted[node.node_id]
                    graph_sorted.append(node)
                    if _include_subdag_tasks and isinstance(node, SubDagOperator):
                        graph_sorted.extend(
                            node.subdag.task_group.topological_sort(_include_subdag_tasks=True)
                        )
            if not acyclic:
                raise AirflowDagCycleException(f"A cyclic dependency occurred in dag: {self.dag_id}")
        return graph_sorted
    def iter_mapped_task_groups(self) -> Iterator[MappedTaskGroup]: