Fix comparison of similarity to threshold in GIST trigram searches.

There was some very strange code here, dating to commit b525bf77, that
purported to work around an ancient gcc bug by forcing a float4 comparison
to be done as int instead.  Commit 5871b884 broke that when it changed
one side of the comparison to "double" but left the comparison code alone.
Commit f576b17c doubled down on the weirdness by introducing a "volatile"
marker, which had nothing to do with the actual problem.

Guess that the gcc bug, even if it's still present in the wild, was
triggered by comparison of float4's and can be avoided if we store the
result of cnt_sml() into a double before comparing to the double "nlimit".
This will at least work correctly on non-broken compilers, and it's way
more readable.

Per bug #14202 from Greg Navis.  Add a regression test based on his
example.

Report: <20160620115321.5792.10766@wrigleys.postgresql.org>