show possible missing branch for unbound var

### Describe the Bug

```
def f(x: bool):
if x:
flag = True
xx = []
else:
flag = False

if flag:
xx.append(1)
```

here xx is "sync" with flag.

But if flow analysis can't figure it out, suggest "else branch" missing an init step might help.

### Sandbox Link

_No response_

### (Only applicable for extension issues) IDE Information

_No response_