By Zara Woodcock Emily Blunt revealed her Oppenheimer co-star Cillian Murphy accidentally ‘smashed his head open’ during a good night’s sleep. The Devils Wear Prada star, 41, joined her co-stars Cillian, 47, and Robert Downey Jr., 58, on Jimmy Kimmel Live! on Thursday evening. At one point, she revealed a special pillow she bought for Irish icon Cillian caused him to suffer a head injury in the middle of the night. She noticed the leading man was in dire need of sleep while filming the award-winning movie about the creation of the atomic bomb. Speaking to host Jimmy, she said: “I just felt Cil…