**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)


self,
        task_or_task_list: DependencyMixin | Sequence[DependencyMixin],
        edge_modifier: EdgeModifier | None = None,
    ) -> None:
        """Set a node (or nodes) to be directly downstream from the current node."""
        self._set_relatives(task_or_task_list, upstream=False, edge_modifier=edge_modifier)
    def set_upstream(
        self,
        task_or_task_list: DependencyMixin | Sequence[DependencyMixin],
        edge_modifier: EdgeModifier | None = None,
    ) -> None:
        """Set a node (or nodes) to be directly upstream from the current node."""
        self._set_relatives(task_or_task_list, upstream=True, edge_modifier=edge_modifier)
    @property
    def downstream_list(self) -> Iterable[Operator]:
        """List of nodes directly downstream"""
        if not self.dag:
            raise AirflowException(f"Operator {self} has not been assigned to a DAG yet")
        return [self.dag.get_task(tid) for tid in self.downstream_task_ids]
    @property
    def upstream_list(self) -> Iterable[Operator]: