The whistle and chant are employed at the Garden, of course, whether or not the opponent is the Islanders. If the Penguins are in town, Potvin sucks. If the Bruins, the Flyers, the Kings, whoever . . . Potvin still sucks. The chant is the password, really, that must be shouted by a fan if he is to prove himself a true believer. The other day at the Garden, opening night, even Tom Hanks mouthed the sacred mantra into a camera. You could see him on the giant scoreboard, saying the words that Bova initiates.