Warn if a value is never or always null

**Version Used**: NullableReferenceTypes branch

**Steps to Reproduce**:
```cs
    public void M(string? o) {
        if (o == null) return;
        var x = o?.Length; //!
    }
```

**Expected Behavior**: warning: `o` is never null

**Actual Behavior**: no warnings

Relates to https://github.com/dotnet/roslyn/issues/25592