Fix S2259 FP: Raising issue in unreachable code when using declaration pattern on unknown value

[Tim-Pohlmann]

Do not fix this before #8326. By coincidence, it is suppressing a lot of FPs for S2589.

void Test(object o, int i)
{
    object n = null;
            
    if (i is var x1)
        Console.WriteLine();
    else
        Console.WriteLine(n.ToString());    // Compliant: unreachable
            
    if (o is var x2)
        Console.WriteLine();
    else
        Console.WriteLine(n.ToString());    // Noncompliant FP: unreachable
}

When the value of an IsPattern has no SymbolicValue, the engine will not learn anything even if the pattern will always/never match.
This behavior is caused by the IsPattern processor and documented in a unit test.