Speed up HLL computation.

Trivial change to turn a mod into an and.

Results on ndv(l_orderkey) on a bigger version of the lineitem table.
Before:
QueryTime: 1.70s

After:
QueryTime: 1.40s

Change-Id: I3fa0dcb4da6efcd8ff1115ad6e5a3bdd990a35df
Reviewed-on: http://gerrit.ent.cloudera.com:8080/2512
Reviewed-by: Nong Li <nong@cloudera.com>
Tested-by: jenkins