Outputting Standard SQL

This is a minor gripe in the grand scheme of things, but I'm a little
annoyed that we accept standard SQL but then don't spit it back out.

For example:

```

EXPLAIN (COSTS OFF) SELECT * FROM pg_am WHERE amname LIKE '%t%';

            QUERY PLAN            
-----------------------------------
 Seq Scan on pg_am
   Filter: (amname ~~ '%t%'::text)
(2 rows)

```

Why don't we convert that back to LIKE?  Sure, if someone actually typed
"~~" instead of "LIKE" then that wouldn't match what they wrote, but I
much prefer differing in that direction than the current one.

I am not advocating we attempt anything more complex such as "x ~>=~ 'y'
AND x ~<~ 'z'", just that we output SQL where feasible.  I would like to
fiddle with this if there is consensus that a decent patch would be
accepted.

--

Vik Fearing