pgsql: In predtest.c, install a limit on the number of branches we will

Log Message:
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In predtest.c, install a limit on the number of branches we will process in
AND, OR, or equivalent clauses: if there are too many (more than 100) just
exit without proving anything.  This ensures that we don't spend O(N^2) time
trying (and most likely failing) to prove anything about very long IN lists
and similar cases.

Also, install a couple of CHECK_FOR_INTERRUPTS calls to ensure that a long
proof attempt can be interrupted.

Per gripe from Sergey Konoplev.

Back-patch the whole patch to 8.2 and just the CHECK_FOR_INTERRUPTS addition
to 8.1.  (The rest of the patch doesn't apply cleanly, and since 8.1 doesn't
show the complained-of behavior anyway, it doesn't seem necessary to work
hard on it.)

Modified Files:
--------------
    pgsql/src/backend/optimizer/util:
        predtest.c (r1.20 -> r1.21)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/util/predtest.c?r1=1.20&r2=1.21)