Expect the iOS 11 Release Date First
Expect iOS 11, the software that will almost certainly power the iPhone 8, to be released before the iPhone 8 release date this fall.
New iOS updates typically roll out to old phones and tablets before Apple releases new iPhones. Last year, iOS 10 was released on September 13th, a few days ahead of the iPhone 7's September 16th release date.
If you want to go back further, iOS 9 was released on September 16th, a few days before the iPhone 6s' release date on September 25th. So there's precedent here and there's no reason to believe Apple will stray too far from its standard release protocol.
If Apple's iPhone 8 release date is September 22nd, we should see the iOS 11 release date arrive somewhere between September 12th and September 21st.