Hit movies have sequels, so why not hit albums? In the last few months, Neil Young released Harvest Moon (a companion to 1972's Harvest). Next month Atlantic Records will follow up its 1990 Led Zeppelin boxed set with a collection of everything that wasn't on the first box. Meat Loaf, meanwhile, is trying to jump-start his career with Bat Out of Hell II: Back Into Hell, a nod to his sole U.S. hit, 1977's 23 million-selling Bat Out of Hell. ''The first record was about the perennial teen,'' says Carol Peters, senior vice president of Loaf's Left Bank Management. ''This record describes the human condition in an ageless way.'' Yeah, sure.