IMPALA-892: Fix subexpr for IR generated from compound predicate.

Change-Id: I638533827e97f3486eb75a571b18f9e8d1cd4aed
Reviewed-on: http://gerrit.ent.cloudera.com:8080/1973
Reviewed-by: Nong Li <nong@cloudera.com>
Tested-by: jenkins
Reviewed-on: http://gerrit.ent.cloudera.com:8080/1956