JENNIFER LOPEZ and STEVEN TYLER are supposedly going to perform on "American Idol" this season, but NBC's new singing competition "The Voice" has the jump on them.  BTW "The Voice" premieres next Tuesday.  "Voice" coaches Christina Aguilera, CEE LO GREEN, Adam Levine and country singer Blake Shelton all got on stage to perform the Gnarls Barkley song "Crazy" together . . . and they rocked it.  Seriously, it's pretty cool.  NBC just released the promo video for it, which you can check out here.