pgsql: Fix some problems with selectivity estimation for partial

Log Message:
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Fix some problems with selectivity estimation for partial indexes.

First, genericcostestimate() was being way too liberal about including
partial-index conditions in its selectivity estimate, resulting in
substantial underestimates for situations such as an indexqual "x = 42"
used with an index on x "WHERE x >= 40 AND x < 50". While the code is
intentionally set up to favor selecting partial indexes when available,
this was too much...

Second, choose_bitmap_and() was likewise easily fooled by cases of this
type, since it would similarly think that the partial index had selectivity
independent of the indexqual.

Fixed by using predicate_implied_by() rather than simple equality checks
to determine redundancy. This is a good deal more expensive but I don't
see much alternative. At least the extra cost is only paid when there's
actually a partial index under consideration.

Per report from Jeff Davis. I'm not going to risk back-patching this,
though.

Modified Files:
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pgsql/src/backend/optimizer/path:
indxpath.c (r1.217 -> r1.218)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/path/indxpath.c.diff?r1=1.217&r2=1.218)
pgsql/src/backend/utils/adt:
selfuncs.c (r1.229 -> r1.230)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/selfuncs.c.diff?r1=1.229&r2=1.230)
pgsql/src/test/regress/expected:
select.out (r1.16 -> r1.17)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/select.out.diff?r1=1.16&r2=1.17)
pgsql/src/test/regress/sql:
select.sql (r1.12 -> r1.13)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/sql/select.sql.diff?r1=1.12&r2=1.13)