It didn't, however. So how to get it back? I knew the iTunesHelper was hidden in the iTunes.app package, so I couldn't access it through Login's Add... button. I originally thought I'd just create an alias to the iTunesHelper and add that, but in the end I didn't need to. Just dragging the app onto the Login Items list is enough.
So, Show iTunes' package contents using the Finder's contextual menu. Navigate to the Resources folder, find iTunesHelper and drag it to the open Login Items pane. Done!

