Enable CS8073 in a warning wave

[gafter]

CS8073 applies in a scenario such as this:

class Program
{
    public static void M(S s)
    {
        if (s == null) { } // CS8073
        if (s != null) { } // CS8073
    }
}
struct S
{
    public static bool operator==(S s1, S s2) => false;
    public static bool operator!=(S s1, S s2) => true;
    public override bool Equals(object other) => false;
    public override int GetHashCode() => 0;
}

(6,13): warning CS8073: The result of the expression is always 'false' since a value of type 'S' is never equal to 'null' of type 'S?'
(7,13): warning CS8073: The result of the expression is always 'true' since a value of type 'S' is never equal to 'null' of type 'S?'

The compiler currently only produces this warning in "strict" mode. As part of the warning wave feature, this warning should be included in warning wave "5".