IMPALA-765: Fix subexpr elimination codegen optimization.

The previous implementation did not properly handle replacing the is_null
return argument from expr calls.

Change-Id: I96cd0dfca8876b4f914b0cbc4eb459ea3dcdf230
Reviewed-on: http://gerrit.ent.cloudera.com:8080/1838
Reviewed-by: Nong Li <nong@cloudera.com>
Tested-by: jenkins