"The Court rests heavily on the premise -- recognized in both Roe and Maher -- that the State's legitimate interest in preserving potential life provides a sufficient justification for funding medical services that are necessarily associated with normal childbirth without also funding abortions that are not medically necessary. The Maher opinion repeatedly referred to the policy of favoring "normal childbirth." See 432 U.S. at 432 U. S. 477, 432 U. S. 478, 432 U. S. 479. But this case involves a refusal to fund abortions which are medically necessary to avoid abnormal childbirth."