IMPALA-1292: Incorrect result in analytic SUM when ORDER BY column is null

The 'less than' predicate created by AnalyticPlanner used to check if the
previous row was less than the current row is not exactly what we want
to determine when rows in RANGE windows (the default window in this case)
share the same result values. Rows get the same results when the order by
exprs evaluate equally or both null, so it's easiest (and more efficient)
to use a predicate that simply checks equality or both null. We already
create such predicates for checking for partition boundaries, so this is
a trivial change.

When we support arbitrary RANGE window offsets we will likely want to
add similar predicates that compare two tuples plus/minus the offset,
but those will be simpler because there can be only one order by expr
when specifying RANGE offsets with PRECEDING/FOLLOWING.

Change-Id: I52ff6203686832852430e498eca6ad2cc2daee98
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/4474
Tested-by: jenkins
Reviewed-by: Matthew Jacobs <mj@cloudera.com>
Tested-by: Matthew Jacobs <mj@cloudera.com>