Fix unnecessary break generation in ConvertToSwitchCodeFix

Fix bug in CC019.

This code:

    if (s == "A")
    {
        DoStuff();
    }
    else if (s == "B")
    {
        DoStuff();
    }
    else if (s == "C")
    {
        DoStuff();
    }
    else
    {
        throw new Exception();
    }


Is converted to:

    switch (s)
    {
        case "A":
            DoStuff();
            break;
        case "B":
            DoStuff();
            break;
        case "C":
            DoStuff();
            break;
        default:
            throw new Exception();
            break;
    }

This causes a "CS0162: Unreachable code detected" after the throw statement.