pgsql: 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

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/518442c7f334f3b05ea28b7ef50f1b551cfcc23e

Modified Files
--------------
src/backend/statistics/extended_stats.c | 86 ++++++++++++++++++++++-----------
src/backend/statistics/mcv.c            |  4 ++
src/test/regress/expected/stats_ext.out | 19 ++++++++
src/test/regress/sql/stats_ext.sql      | 19 ++++++++
4 files changed, 100 insertions(+), 28 deletions(-)