pgsql: Undo faulty attempt at not relying on RINFO_IS_PUSHED_DOWN.

Undo faulty attempt at not relying on RINFO_IS_PUSHED_DOWN.

I've had a bee in my bonnet for some time about getting rid of
RestrictInfo.is_pushed_down, because it's squishily defined and
requires not-inexpensive extra tests to use (cf RINFO_IS_PUSHED_DOWN).
In commit 2489d76c4, I tried to make remove_rel_from_query() not
depend on that macro; but the replacement test is buggy,
as exposed by a report from Rushabh Lathia and Robert Haas.
That change was pretty incidental to the main goal of 2489d76c4,
so let's just revert it for now.  (Getting rid of is_pushed_down
is still far away, anyway.)

Discussion: https://postgr.es/m/CA+TgmoYco=hmg+iX1CW9Y1_CzNoSL81J03wUG-d2_3=rue+L2A@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/c8b881d21fd8e0214bc43280d6231cc2c6ef55ef

Modified Files
--------------
src/backend/optimizer/plan/analyzejoins.c |  7 +------
src/test/regress/expected/join.out        | 16 ++++++++++++++++
src/test/regress/sql/join.sql             |  6 ++++++
3 files changed, 23 insertions(+), 6 deletions(-)