IMPALA-932: evaluate concat/concat_ws children once

Change-Id: Id22a6c1dfb57cf659a1e24af4de6e5a2336cafa4
Reviewed-on: http://gerrit.ent.cloudera.com:8080/2152
Reviewed-by: Skye Wanderman-Milne <skye@cloudera.com>
Tested-by: jenkins