[Nullable] == comparing against a non-null value should inform the null state

[gafter]

See HERE.

In the nullable reference types branch, the compiler produces four warnings on the following code, only two of which are deserved.

using System;
public class C {
    public static void Main()
    {
        A a = new A();
        if (a.B == "")
        {
            a.B.ToString(); // improper warning
        }
        else
        {
            a.B.ToString(); // warning (OK)
        }
        
        A? a2 = null;
        if (a2 == A.Instance)
        {
            a2.ToString(); // improper warning
        }
        else
        {
            a2.ToString(); // warning (OK)
        }
    }
}

class A
{
    public static A Instance => new A();
    public string? B = M();
    static string? M() => null;
}

[gafter]

Dup of #27928