“ But it does not follow that the Court of Appeals, in the face of the Commission's insistence that its decision was wholly consistent with its Cities Service formula, should have itself determined that consolidated return savings be first allocated to nonjurisdictional income and that 'income from the unregulated component of United is sufficiently large to absorb all such net tax losses and no excess remains to reduce the regulated taxable income of United.' United Gas Pipe Line Co. ”