IMPALA-904: Make TupleIsNullPredicate work on non-nullable tuples.

We wrap certain exprs substituted from outer-joined inline view in an expr that
evaluates to NULL if the underling tuple(s) are NULL. We do this for exprs that evaluate
to non-NULL values if their slots are NULL, i.e., we must then distinguish tuples that are
NULL from slots that are NULL (otherwise evaluating an expr against a tuple that is NULL
due to the outer join may incorrectly return a non-NULL value.)

The bug: Exprs referring to an outer-joined inline view may appear in various places
in the outer query block. For example, they could appear in an On-clause or be
placed into scans/aggregates due to predicate propagation. In such cases, the underlying
tuples may not be nullable yet because they only become nullable after the outer join.
We had a DCHECK in tuple-is-null-predicate.cc requiring the tuples to be nullable.
The fix: Remove the DCHECK. The fix is not elegant but practical. It would be rather
difficult to fix the inline view expr substitution such that a TupleIsNullPredicate
never references a non-nullable tuple, esp. due to predicate propagation.

Change-Id: I180f75f14173f356abfeec751e6b2d419378a9a7
Reviewed-on: http://gerrit.ent.cloudera.com:8080/2146
Reviewed-by: Alex Behm <alex.behm@cloudera.com>
Reviewed-by: Marcel Kornacker <marcel@cloudera.com>
Tested-by: jenkins