**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)


"""Tests the >> / << style with a plain operator"""
        
        dag, (op1, op2, op3, op4) = test_dag
        
        op1 >> Label("Label 1") >> op2
        op3 << Label("Label 2") << op2 >> op4
        
        assert dag.get_edge_info(op1.task_id, op2.task_id) == {"label": "Label 1"}
        assert dag.get_edge_info(op2.task_id, op3.task_id) == {"label": "Label 2"}
        assert dag.get_edge_info(op2.task_id, op4.task_id) == {}
    def test_tasklist_shift(self, test_dag):
        """Tests the >> / << style with a list of operators"""
        
        dag, (op1, op2, op3, op4) = test_dag
        
        op1 >> Label("Label 1") >> [op2, op3] << Label("Label 2") << op4
        
        assert dag.get_edge_info(op1.task_id, op2.task_id) == {"label": "Label 1"}
        assert dag.get_edge_info(op1.task_id, op3.task_id) == {"label": "Label 1"}
        assert dag.get_edge_info(op4.task_id, op2.task_id) == {"label": "Label 2"}
    def test_xcomarg_shift(self, test_dag):