`del` in dead code should make variable local

Even though `del` is in unreachable code (`if False`), Python's compiler still marks the variable as local in the enclosing scope. Pyrefly should recognize this and report that `x` may be unbound.

```python
x = 1

def f():
print(x)
if False:
del x
```

**Sandbox:** [Pyrefly](https://pyrefly.org/sandbox/?project=N4IgZglgNgpgziAXAAlHAhgOwCYCMD2AHgHQAOAnksiIcgLzICMAOpq9jGMmABQCUiVsmHJSAJwiYALj0J8hIiFwBi6KHBiDMIncg5RkhViAC%2BAGmroAxlIgA3GMugwqIDDgIkKpoA)