Fixes for ChangeVarNodes_walker()

Fixes for ChangeVarNodes_walker()

This commit fixes two bug in ChangeVarNodes_walker() function.

 * When considering RestrictInfo, walk down to its clauses based on the
   presense of relid to be deleted not just in clause_relids but also in
   required_relids.

 * Incrementally adjust num_base_rels based on the change of clause_relids
   instead of recalculating it using clause_relids, which could contain
   outer-join relids.

Reported-by: Richard Guo <guofenglinux@gmail.com>
Discussion: https://postgr.es/m/CAMbWs49PE3CvnV8vrQ0Dr%3DHqgZZmX0tdNbzVNJxqc8yg-8kDQQ%40mail.gmail.com
Author: Andrei Lepikhov <lepihov@gmail.com>
Reviewed-by: Alexander Korotkov <aekorotkov@gmail.com>