The gossip has gone batty. A week before Batman and Robin begins shooting, another rumor has taken wing. Director Joel Schumacher is battling talk that Liv Tyler is replacing Batgirl Alicia Silverstone. That's about as true as Schumacher's other favorite false Bat reports: ''Patrick Stewart is playing Mr. Freeze. [Arnold Schwarzenegger is.] And Demi Moore is playing Poison Ivy. No, Julia Roberts is Poison Ivy. No, Sharon Stone is. [Uma Thurman got the part.] Tim Burton is shooting Catwoman with the same cast and set. [He's not.] And Jim Carrey is making a surprise appearance. [Sorry, no.]'' With a buildup like that, we can't wait to hear what happens when the cast, including George Clooney and Chris O'Donnell (no, really), reports to work Sept. 12.