pgsql: Fix subselect.c to avoid assuming that a SubLink's testexpr

Log Message:
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Fix subselect.c to avoid assuming that a SubLink's testexpr references each
subquery output column exactly once left-to-right. Although this is the case
in the original parser output, it might not be so after rewriting and
constant-folding, as illustrated by bug #3882 from Jan Mate. Instead
scan the subquery's target list to obtain needed per-column information;
this is duplicative of what the parser did, but only a couple dozen lines
need be copied, and we can clean up a couple of notational uglinesses.
Bug was introduced in 8.2 as part of revision of SubLink representation.

Tags:
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REL8_2_STABLE

Modified Files:
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pgsql/src/backend/optimizer/plan:
subselect.c (r1.112.2.2 -> r1.112.2.3)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/plan/subselect.c?r1=1.112.2.2&r2=1.112.2.3)