We all know that Johnny Rzeznik is one sexy dude. The ruggedly handsome Goo Goo Dolls frontman hides behind mirrored shades, has tattoos in all the right places and sports the best rock-star mane that money can buy. Pop- music fans flock to his band’s concerts to hear him sing such Top 40 gems as “Iris,” “Name” and “Slide.” How unexpected then that Johnny Cool was upstaged Tuesday night at a near-capacity Bank of America Pavilion by his bass player, the insanely overenthusiastic Robby Takac. Short, slightly pudgy and with greasy, coal-black hair obscuring his face, Takac bounc