Messy San Franciscan garage punk Ty Segall is an unlikely recent entrant into the late-night musical-guest roundtable, and his thrashed-out tantrums will always look pretty incongruous on soundstages. Last night, he had David Letterman grinning with amusement when he and his band bashed out a raucous version of his Twins song "You're The Doctor," which ended when he screeched "go ...