When flow->hashExpr is created, we ignore the relable node while
checking the existence of the expression in the targetlist, if a match
is not found in the targetlist, hashExpr is added to the targetlist with
resjunk=true.

In create_join_plan
```
  /**
   * If plan has a flow node, ensure all entries of hashExpr
   * are in the targetlist.
   */
  if (plan->flow && plan->flow->hashExpr)
  {
  	plan->targetlist = add_to_flat_tlist(plan->targetlist, plan->flow->hashExpr, true /* resjunk */ );
  }
```
add_to_flat_tlist uses tlist_member_ignore_relabel
Later, in set_plan_refs, the references of the vars are updated. This
commit ensures that the contract of ignoring the relable node is
held while trying to update the references in hashExpr. Prior to this
commit, in set_plan_refs while checking the existing of hashExpr in
targetlist relable node were not ignored which resulted in not finding
the coressponding projection element and the planning used to fail with
ERROR: variable not found in subplan target list

fix_upper_expr ensures that the cast/type of the variable in the upper
plan node is not ignore, thus it considered a plain match for non vars
expression. Thus, added a new method to perform special handling of
flow->hashExpr as they have to be matched with the target list at the
same level (not of the child).

This has been a bug in 5 and previous version of greenplum since long,
this is a greenplum specific fix, so may be was never caught.