Can/should `is` operator update null-state in false case?

```C#
string? s = null;
if (!(s is object)) { s.ToString(); /* could warn? */ }
if (!(s is string)) { s.ToString(); /* could warn? */ }
```

There are variants of this scenario with `string!` and `string~`. The question is whether we should learn in both branches, or whether we should treat one branch as unreachable.

Relates to https://github.com/dotnet/roslyn/pull/30283