IMPALA-888: Drop union operands with constant conjuncts evaluating to false.

This patch simplifies the complex slot materialization logic for unions by
making the materialization independent of conjuncts assigned to MergeNodes.
When 'pushing down' predicates into union operands, we drop union operands
with constant predicates evaluating to false. Constant predicates that
evaluate to true are simply ignored.

Change-Id: I0e7ccfb206bed29db2b5d667e2bb61310980e80a
Reviewed-on: http://gerrit.ent.cloudera.com:8080/2270
Reviewed-by: Alex Behm <alex.behm@cloudera.com>
Tested-by: jenkins