Handle self-assignment for walrus operator

### Describe the Bug

Minimal repro:
```python
x = 0
(x := x + 1)
reveal_type(x)
```
Expected: Reveals `Literal[1]` or `int`
Actual: Reveals `Unknown`

### Sandbox Link

https://pyrefly.org/sandbox/?code=B4AgvCAMBQAUoC4KgNQgIwEpoCcCmAbngIYA2A+gC4CeADnvJkA

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

_No response_