Make ExecInitExpr build the list of SubPlans found in a plan tree in order

Make ExecInitExpr build the list of SubPlans found in a plan tree in order
of discovery, rather than reverse order.  This doesn't matter functionally
(I suppose the previous coding dates from the time when lcons was markedly
cheaper than lappend).  However now that EXPLAIN is labeling subplans with
IDs that are based on order of creation, this may help produce a slightly
less surprising printout.