pgsql: Improve hash method for bitmapsets: some examination of actual

Log Message:
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Improve hash method for bitmapsets: some examination of actual outputs
shows that adding a circular shift between words greatly improves the
distribution of hash outputs.

Modified Files:
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pgsql/src/backend/nodes:
bitmapset.c (r1.8 -> r1.9)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/nodes/bitmapset.c.diff?r1=1.8&r2=1.9)