Do not translate dummy SpecialJoinInfos for child joins

Do not translate dummy SpecialJoinInfos for child joins

This teaches build_child_join_sjinfo() to create the dummy
SpecialJoinInfos (those created for inner joins) directly for a given
child join, skipping the unnecessary overhead of translating the
parent joinrel's SpecialJoinInfo.

To that end, this commit moves the code to initialize the dummy
SpecialJoinInfos to a new function named init_dummy_sjinfo() and
changes the few existing sites that have this code and
build_child_join_sjinfo() to call this new function.

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