Watching NBC's Saturday Night Live in past seasons, it was possible to dismiss Dennis Miller as a smug twit, his smirk exceeded only by the length of his shaggy hair. This season, however, Miller has changed; it's not just that his hair is shorter he's given his comedy a make-over as well. The political jokes in his ''Weekend Update'' have become bitter and blunt, and in his new HBO special, Dennis Miller: Black & White, Miller goes even further-from bluntness to a dazzling pop-cultural complexity. The hour offers Miller doing his latest stand-up act to a nightclub audience. But he's working on a level the nightclub crowd hasn't seen since Mort Sahl in his prime this is the most recondite comedy act around. It's impossible to pin Miller down politically he snaps at George Bush and animal-rights activists, and comes out in favor of the death penalty and against born-again Christian ministers (''I can see through these guys like used Neutrogena''). Jokes are made about Herman Melville, Bachman-Turner Overdrive, Marion Barry. Near the end, he yells, ''Stop me before I cross- reference again!'' Not on your life, Dennis more, more.