WASHINGTON — The Supreme Court on Tuesday agreed to decide whether states and local governments can ban semiautomatic rifles like the AR-15, which are popular among gun enthusiasts but have also been used in high-profile mass shootings. The court, which has a 6-3 conservative majority that generally backs gun rights, will hear challenges to laws in Connecticut and Cook County, Illinois, which covers the Chicago area. The two combined cases will be argued and decided in the court’s next term,