Allow usage of match_orclause_to_indexcol() for joins

Allow usage of match_orclause_to_indexcol() for joins

This commit allows transformation of OR-clauses into SAOP's for index scans
within nested loop joins.  That required the following changes.

 1. Make match_orclause_to_indexcol() and group_similar_or_args() understand
    const-ness in the same way as match_opclause_to_indexcol().  This
    generally makes our approach more uniform.
 2. Make match_join_clauses_to_index() pass OR-clauses to
    match_clause_to_index().
 3. Also switch match_join_clauses_to_index() to use list_append_unique_ptr()
    for adding clauses to *joinorclauses.  That avoids possible duplicates
    when processing the same clauses with different indexes.  Previously such
    duplicates were elimited in match_clause_to_index(), but now
    group_similar_or_args() each time generates distinct copies of grouped
    OR clauses.

Discussion: https://postgr.es/m/CAPpHfdv%2BjtNwofg-p5z86jLYZUTt6tR17Wy00ta0dL%3DwHQN3ZA%40mail.gmail.com
Reviewed-by: Andrei Lepikhov 
Reviewed-by: Alena Rybakina 
Reviewed-by: Pavel Borisov