If, even after a stroke, you'd still be able to sing the entire score of Assassins, or if you think God looks something like Elaine Stritch, Shrek the Musical (through September 21 at the 5th Avenue Theatre, tickets $28-$90) is probably not your cup of tea. We expected to be solidly in that camp at the opening night world premiere of the DreamWorks-based musical. Imagine our surprise when we loved it--and loved the bad guy......