pgsql: Fix scalararraysel() to cope with binary-compatible cases, such

Log Message:
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Fix scalararraysel() to cope with binary-compatible cases, such as text[]
versus varchar[].  This oversight probably explains Ryan Holmes' recent
complaint --- he was getting a generic selectivity estimate instead of
anything intelligent.

Tags:
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REL8_2_STABLE

Modified Files:
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    pgsql/src/backend/utils/adt:
        selfuncs.c (r1.214.2.2 -> r1.214.2.3)

(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/selfuncs.c.diff?r1=1.214.2.2&r2=1.214.2.3)