Report unreachable when a sub-expression is `Never`

```py
print(exit())
print("hi") # error: unreachable
```
I would expect the first `print` to also be unreachable

Start by running the reported Python example through basedmypy and compare the unreachable diagnostics for both print calls. Trace the checker’s unreachable-code analysis for expressions containing Never; done means the first print is also reported as unreachable while the existing diagnostic remains correct.

Written by the indexing model from the issue text.