Regression in 1.7: mypy loses truthiness constraint for union containing tuple

```
def f(x: bool | tuple[int, int]):
    if x:
        if x is True or x[0]:
            pass
```
Gives:
```
main.py:3: error: Value of type "Literal[False] | tuple[int, int]" is not indexable  [index]
```

This error is newly reported after #16237