SPRINGFIELD — The U.S. Constitution’s ban against “cruel and unusual punishment” has been wielded frequently to strike down lengthy sentences for juveniles.But an overlooked Illinois constitutional requirement that says offenders should get a shot at redemption may have a similar effect on lengthy sentences for adults.That possibility may have increased this morning as the state’s high court agreed to take up a case in which an 18-year-old’s 76-year prison sentence was ruled to have …