New analyzer: unreliable null check

[ronnygunawan]

A null check is unreliable when:

1. Checked variable is a mutable field or a mutable property
2. The value is read again after the null check
3. No new local was introduced before the value is read again.
4. Value is written somewhere else

class C {
    string? S;
    List<string> L = new();
    void M() {
        if (S is null) return;
        L.Add(S); // Null check was unreliable
    }
    void N() {
        S = null;
    }
}