Transform OR-clauses to SAOP's during index matching

Transform OR-clauses to SAOP's during index matching

This commit makes match_clause_to_indexcol() match
"(indexkey op C1) OR (indexkey op C2) ... (indexkey op CN)" expression
to the index while transforming it into "indexkey op ANY(ARRAY[C1, C2, ...])"
(ScalarArrayOpExpr node).

This transformation allows handling long OR-clauses with single IndexScan
avoiding diving them into a slower BitmapOr.

We currently restrict Ci to be either Const or Param to apply this
transformation only when it's clearly beneficial.  However, in the future,
we might switch to a liberal understanding of constants, as it is in other
cases.

Discussion: https://postgr.es/m/567ED6CA.2040504%40sigaev.ru
Author: Alena Rybakina, Andrey Lepikhov, Alexander Korotkov
Reviewed-by: Peter Geoghegan, Ranier Vilela, Alexander Korotkov, Robert Haas
Reviewed-by: Jian He, Tom Lane, Nikolay Shaplov