LAS VEGAS — Conor McGregor’s opening punch bloodied Donald Cerrone’s nose. He then floored Cerrone only 20 seconds into the bout with a perfectly placed kick to the head, and he mercilessly finished on the ground. When he paraded around the ring with an Irish flag on his shoulders to celebrate, the mixed martial arts world knew McGregor is back with a big bang. The Irish former two-division champion came out of a three-year stretch of relative inactivity and outside-the-cage troubles with a welterweight performance in UFC 246 on Saturday night that echoed his greatest fights during his unparalleled rise. “I feel really good, and I came out of here unscathed,” McGregor said.