Miley Cyrus broke down crying in Vegas yesterday ... while she performed a song rumored to be about her break up with Liam Hemsworth. Miley twerked it up on stage at the iHeartRadio Music Festival -- but the tears started flowing hard once she began singing "Wrecking Ball." The couple has called it quits ... and Liam has gotten tight with telenovela actress Eiza Gonzales -- who BTW is crazy hot. Despite the cry-fest ... Miley cleaned up for her second performance of the night and wore yet another ensemble that left very little to the imagination. And wore this.