IMPALA-888: Materialize union slots referenced by constant predicates.

To keep the predicate assignment/propagation logic simple, we assign conjuncts
whose underlying base table exprs are constant in at least one union operand
to the evaluating MergeNode, and not in the operand(s) whose corresponding base
table exprs are constant.
The JIRA describes two different bugs:
The first bug was that the slots required for evaluating such predicates in the
MergeNode were not marked as materialized. The second bug was that predicates
'pushed' into union operands did not get re-analyzed after substituting the
predicate's exprs with the result exprs of that union operand. Missing casts
lead to a crash. The new test covers both bugs.

Change-Id: I0f5b8a366b32f7d4b2587e13793b6103cdf7e8b3
Reviewed-on: http://gerrit.ent.cloudera.com:8080/2135
Reviewed-by: Alex Behm <alex.behm@cloudera.com>
Tested-by: jenkins