Will c# compiler optimize empty if blocks

Well you can use ildasm to see what the compiler has optimized for yourself. But if you were expecting it to remove the code entirely, it can't - because those three method calls could throw exceptions or modify state. So the best it could do would be to emit the equivalent of:

if (specField != null)
{
    if (!AddSystemType(specField, layout))
    {
        if (!AddEnumType(specField, layout))
        {
            AddUserType(specField, layout);
        }
    }
}