Tweak tsmatchsel() so that it examines the structure of the tsquery whenever

Tweak tsmatchsel() so that it examines the structure of the tsquery whenever
possible (ie, whenever the tsquery is a constant), even when no statistics
are available for the tsvector.  For example, foo @@ 'a & b'::tsquery
can be expected to be more selective than foo @@ 'a'::tsquery, whether
or not we know anything about foo.  We use DEFAULT_TS_MATCH_SEL as the assumed
selectivity of individual query terms when no stats are available, then
combine the terms according to the query's AND/OR structure as usual.

Per experimentation with Artur Dabrowski's example.  (The fact that there
are no stats available in that example is a problem in itself, but
nonetheless tsmatchsel should be smarter about the case.)

Back-patch to 8.4 to keep all versions of tsmatchsel() in sync.