Section 81GG. Any planning board having powers of subdivision control under corresponding provisions of earlier laws shall have all of the powers and be subject to all of the duties of a planning board with respect to subdivision control under sections eighty-one K to eighty-one FF, inclusive, without any further action by such city or town. If any provision of sections eighty-one K to eighty-one GG, inclusive, known as the subdivision control law, or in the administration thereof, shall be held to be unconstitutional, it shall not affect any other provision of said sections or the administration thereof.