There is code for creating objects to back map entries that is duplicated, and this change seeks to eliminate that duplication. Also, the calls to the clip_start function follow a common pattern, in most cases, and by making the pattern into a function, vm_map_lookup_clip_start, I can drop some temporary variables in various places.
Later, I'd like to rewrite vm_map_lookup_clip_start to combine some of the map operations that it uses, to access entries less often and make the whole thing more efficient.