Ever since the instance domain was added, this was exposed, it just didn't do anything. This patch implements the instances domain in the delete and separate geometry nodes, where it acts on the top-level instances. We act on a mutable instances input, with the idea that eventually copy on write attribute layers will make this less expensive. It also allows us to keep the instance references in place and to do less work in some situations. Ref T93554 Differential Revision: https://developer.blender.org/D13565