Only retrieve a mutable copy of the attribute if we're actually able to change it.
If topology changes and there are no IDs, we can't mix the attribute and we
should avoid retrieving it in case it's shared. This is more of a hypothetical
change, I didn't actually observe a real world performance change.
Pull Request: https://projects.blender.org/blender/blender/pulls/129811
88f945d068