Chicago — the soft-rock gods who ruled the ’70s airwaves — almost left one of their definitive hits behind.The year was 1976, the band was working on “Chicago X,” and then-lead singer and bassist Peter Cetera had this one track — “If You Leave Me Now” — he really believed in, even if everybody else needed convincing.“[He] had a hard time getting that song on the album,” founding Chicago member Robert Lamm, 78, told The Post about the Cetera-penned tune that took the troupe of musicians in a radically different direction.“We were rocking and jazzing, and then that song was the softest, most beautiful ballad that you could think of. It went No.