Fix handling of clauses incompatible with extended statistics

Fix handling of clauses incompatible with extended statistics

Handling of incompatible clauses while applying extended statistics was
a bit confused - while handling a mix of compatible and incompatible
clauses it sometimes incorrectly treated the incompatible clauses as
compatible, resulting in a crash.

Fixed by reworking the code applying the selected statistics object to
make it easier to understand, and adding a proper compatibility check.

Reported-by: David Rowley
Discussion: https://postgr.es/m/CAApHDvpYT10-nkSp8xXe-nbO3jmoaRyRFHbzh-RWMfAJynqgpQ%40mail.gmail.com