Fix an oversight in cbc127917 to handle MERGE correctly

Fix an oversight in cbc127917 to handle MERGE correctly

ExecInitModifyTable() forgot to trim MERGE-related lists to exclude
entries for result relations pruned during initial pruning, so fix
that.

While at it, make the function's use of the pruned resultRelations
list, rather than ModifyTable.resultRelations, more consistent.

Reported-by: Alexander Lakhin <exclusion@gmail.com> (via sqlsmith)
Reviewed-by: Junwang Zhao <zhjwpku@gmail.com>
Discussion: http://postgr.es/m/e72c94d9-e5f9-4753-9bc1-69d72bd54b8a@gmail.com