CAN WE TALK? Don't invite Cher and Joan Rivers to the same party. The superdivas went tête-à-tête at the New York kickoff of Giorgio Armani's new perfume, Gio. Here's how it went. Rivers: "How come you've done all the big talk shows, like Oprah and Phil, but not mine?" Cher (who added biker touches to her elegant black velvet Armani jacket): ''Because your show is so trashy.'' Rivers: ''But look at how you're dressed! Who are you to talk?'' Apparently, Cher was miffed that gossip columnists on Rivers' show had spoken irreverently about her sex life. Neither side was available for comment.