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