Nothing contained in this part shall be deemed to modify or supplant any provision of any special or private statute providing for a municipal planning commission; and the provisions of any such statute relating to the organization and powers of such commission and all other provisions of any such special or private statute shall remain in full force and effect; but insofar as this chapter grants powers to municipal planning commissions not contained in such special statute, the planning commission created and acting under such special statute shall be deemed to have the additional powers granted in this chapter.