Xcode 9 has changed the way we switch branch.
The branches no more show in source control menu.
Xcode 9 now makes it even simpler for us to manage branches and tags right from the Xcode 9 left panel. The below image contains Xcode 9 source control revealed.
You can see that left panel is exposed and clicking on the highlighted blue icon right to the folder icon on left panel revels the source control with all branches and tags