Introduce join_info_array for direct lookups of SpecialJoinInfo by ojrelid

When working on the improper qual pushdown issue [1], there is a need in
the proposed fix to avoid scanning all the SpecialJoinInfos, since that
is too expensive. I think this might be a common requirement. In the
current codes there are several places where we need to scan all the
SpecialJoinInfos in join_info_list looking for SpecialJoinInfos that
belong to a given outer join relid set, which is an O(n) operation. So
start a new thread for this requirement.

To improve the O(n) operation, introduce join_info_array to allow direct
lookups of SpecialJoinInfo by ojrelid. This is doable because for each
non-zero ojrelid there can only be one SpecialJoinInfo. This can
benefit clause_is_computable_at() and have_unsafe_outer_join_ref(), as
the patch does, and more future usages such as
add_outer_joins_to_relids() in the proposed patch for issue [1].

[1]
https://www.postgresql.org/message-id/flat/0b819232-4b50-f245-1c7d-c8c61bf41827%40postgrespro.ru

Thanks
Richard