Reduce memory used by partitionwise joins

Specifically, this commit reduces the memory consumed by the
SpecialJoinInfos that are allocated for child joins in
try_partitionwise_join() by freeing them at the end of creating paths
for each child join.

A SpecialJoinInfo allocated for a given child join is a copy of the
parent join's SpecialJoinInfo, which contains the translated copies
of the various Relids bitmapsets and semi_rhs_exprs, which is a List
of Nodes.  The newly added freeing step frees the struct itself and
the various bitmapsets, but not semi_rhs_exprs, because there's no
handy function to free the memory of Node trees.

Author: Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>
Reviewed-by: Richard Guo <guofenglinux@gmail.com>
Reviewed-by: Amit Langote <amitlangote09@gmail.com>
Reviewed-by: Andrey Lepikhov <a.lepikhov@postgrespro.ru>
Reviewed-by: Tomas Vondra <tomas.vondra@enterprisedb.com>
Discussion: https://postgr.es/m/CAExHW5tHqEf3ASVqvFFcghYGPfpy7o3xnvhHwBGbJFMRH8KjNw@mail.gmail.com

src/include/nodes/pathnodes.h

@@ -2856,6 +2856,9 @@ typedef struct PlaceHolderVar
  * cost estimation purposes it is sometimes useful to know the join size under
  * plain innerjoin semantics.  Note that lhs_strict and the semi_xxx fields
  * are not set meaningfully within such structs.
+ *
+ * We also create transient SpecialJoinInfos for child joins during
+ * partiotionwise join planning, which are also not present in join_info_list.
  */
 #ifndef HAVE_SPECIALJOININFO_TYPEDEF
 typedef struct SpecialJoinInfo SpecialJoinInfo;