IMPALA-1282: Identical select-list aliases pointing to the same expr are not ambiguous.

Change-Id: If2d395620b1a077d128987b3d139fb5ef1b95e30
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/4434
Reviewed-by: Alex Behm <alex.behm@cloudera.com>
Tested-by: jenkins