Adding Codegen function and tests for CASE expressions.

Change-Id: Ib52b3e3f12b35e2c0a60ef94501c20ef83abdfe5
Reviewed-on: http://gerrit.ent.cloudera.com:8080/3187
Reviewed-by: Paden Tomasello <paden.tomasello@cloudera.com>
Tested-by: jenkins
Reviewed-on: http://gerrit.ent.cloudera.com:8080/3498