Noticed this while looking at compiled code density. In C1 PatchingStub code, we _always_ perform a runtime call to Thread::current(), even though we can rely on r15_thread to be available. This is because we are calling to `MacroAssembler::get_thread`, which is always doing slowpath.

This kind of accident would be less likely / impossible once we cleanup uses of `MacroAssembler::get_thread` with JDK-8353174.