Detect and merge common tails

* src/pf/optimize.lua (simplify_if): Handle more cases.  Detect and
  merge common tails.  (This might not be a good idea, as it decreases
  precision of flow analysis; dunno.)

* doc/host-127.0.0.1.md:
* doc/net-127.0.0.0-8.md: Regenerate.