pgsql: Fix get_sort_group_operators() so that it doesn't think arrays

Log Message:
-----------
Fix get_sort_group_operators() so that it doesn't think arrays can be grouped
via hashing.  Eventually we ought to make that possible, but it won't happen
for 8.4.  Per yesterday's report from Robert Haas.

Modified Files:
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    pgsql/src/backend/parser:
        parse_oper.c (r1.108 -> r1.109)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/parse_oper.c?r1=1.108&r2=1.109)