It used to be that hitting the refresh arrow/loop button in preferences -> account would pull down a new provisioning profile with all ur new devices.
Now it just do
In my case nothing listed above helped on Xcode 8.
Here are the steps I tried, which eventually worked to allow Xcode to fix the problem by once again showing me the Register Device button:
Sidenote: In my case I think the problem was that my login expired (Xcode told me this in Preferences > Accounts). But even after clicking on Sign In Again nothing worked. I think maybe all I had to do to refresh it was sign in again and restart Xcode for the Register Device(s) button to show.