vm_map_simplify_entry considers merging an entry with its two
neighbors, and is used in a way so that if entries a and b cannot be
merged, we consider them twice, first not-merging a with its successor
b, and then not-merging b with its predecessor a. This change replaces
vm_map_simplify_entry with vm_map_try_merge_entries, which compares
two adjacent entries only, and uses it to avoid duplicated
merge-checks.

Tested by: pho
Reviewed by: alc
Approved by: markj (implicit)
Differential Revision: https://reviews.freebsd.org/D20814