Allow MergeNode with constant selects to run correctly on multiple fragment instances.

Change-Id: I0b1ff27f591366b960aa944fadabbb4b35f4b9b4
Reviewed-on: http://gerrit.ent.cloudera.com:8080/2832
Reviewed-by: Alex Behm <alex.behm@cloudera.com>
Tested-by: jenkins