Feature request: Local variable never read

It'd be useful to track if a local variable was read before assigned again:
```python
def main():
x = 5
x = 6
print(x)
```
A common use-case is simply typos or using `if` instead of `elif`:
```python
if case1:
x = 1
if case2:
x = 2
else:
x = 3
```
Where `x = 1` is never used.
The correct snippet after fixing the mistake would be:
```python
if case1:
x = 1
elif case2:
x = 2
else:
x = 3
```

Start by comparing the two Python examples and define when an assignment is considered unused before the variable is reassigned. Done means the requested cases are detected without flagging the final value that is later read, with coverage for both the sequential assignments and the if/elif control flow.

Written by the indexing model from the issue text.