Tidy up vm_map_simplify_entry() and its recently introduced helper
functions. Notably, reflow the text of some comments so that they
occupy fewer lines, and introduce an assertion in one of the new
helper functions so that it is not misused by a future caller.

In collaboration with: Doug Moore <dougm@rice.edu>
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D17635