Cache the most recent region lookup.

Cache the most recent region lookup.

The general value of this is debatable, because access could be pretty
random across a large number of regions.  But the sort code, for example,
likes to ask about chunks of memory that were just recently allocated
or are about to be freed, so this will help there, too.  And it helps
a lot if someone (say) builds a linked list and then frees the elements
in order.