Nirvana’s “Nevermind,” the album that steamrolled pop music in 1991, turns 20 years old this weekend. To celebrate, Universal Music will issue a deluxe edition that promises to pour a fresh layer of cement atop the mythology of “Nevermind” as the Last Album That Changed the World. It’s not. Kurt Cobain, Dave Grohl and Krist Novoselic blew America’s unwashed hair back when Nirvana’s major-label debut arrived on Sept. 24, 1991, but “Nevermind” wasn’t the last album to profoundly change popular music.