pgsql-server: Fix estimate_num_groups to be able to use expression-index

Log Message:
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Fix estimate_num_groups to be able to use expression-index statistics
when there is an expressional index matching a GROUP BY item.

Modified Files:
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pgsql-server/src/backend/utils/adt:
selfuncs.c (r1.165 -> r1.166)
(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/backend/utils/adt/selfuncs.c.diff?r1=1.165&r2=1.166)