IMPALA-1296: Fix DCHECK failure for unnecessary buffered tuple in AnalyticNode

The AnalyticEvalNode had a DCHECK that expected the buffered tuple to
only be set when it was needed (i.e. when there are partition exprs or
order by exprs). However, the FE creates a buffered tuple for an entire
sort group when any AnalyticEvalNodes in the sort group need it and that
tuple is set for all nodes. This reverses the logic so that we DCHECK
the buffered tuple is set when it is needed.

Change-Id: If54b303bc439f235da06a542b46a35c61da9e1bd
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/4489
Reviewed-by: Matthew Jacobs <mj@cloudera.com>
Tested-by: jenkins