INDIANAPOLIS — Trevor Siemian crunched his eyes together and flung his head back as trainers prodded around his battered left shoulder, a picture of pain that has become a common illustration of Thursday nights in the NFL. The frustrating second season as a starting quarterback for Siemian had taken another miserable turn moments earlier, when the Broncos’ quarterback was dragged down by Indianapolis linebacker Barkevious Mingo and suffered a potentially serious shoulder injury.