IMPALA-1165: Predicate dropped: Inline view plus distinct aggregate in
outer query

This commit fixes a bug with marking propagated predicates as assigned
during slot materialization. When gathering predicates for the purpose
of slot materialization, no predicates should be marked as assigned.

Change-Id: I5277dc622990b1731db1ecb3ea646e7b72d4e3db
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/4496
Reviewed-by: Dimitris Tsirogiannis <dtsirogiannis@cloudera.com>
Tested-by: jenkins