Imitation remains the sincerest form of flattery. In "Obsession," Brian de Palma didn't bother hiding his debt to Hitchcock's "Vertigo," in content as well as style. Now, in Dressed to Kill, de Palma cheerfully rips off "Psycho" and moments of "North by Northwest," including bits of full-color gore that Hitchcock never would have approved. Much of the movie is harmless fun. But the story eventually gets swamped by its own sex-and-violence grotesque . . .