pgsql: Skip system attributes when applying mvdistinct stats

Skip system attributes when applying mvdistinct stats

When estimating number of distinct groups, we failed to ignore system
attributes when matching the group expressions to mvdistinct stats,
causing failures like

ERROR: negative bitmapset member not allowed

Fix that by simply skipping anything that is not a regular attribute.
Backpatch to PostgreSQL 10, where the extended stats were introduced.

Bug: #16111
Reported-by: Tuomas Leikola
Author: Tomas Vondra
Backpatch-through: 10
Discussion: https://postgr.es/m/16111-687799584c3a7e73@postgresql.org

Branch
------
REL_10_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/0b0f281cc0fe383162b170889e79502e21e6b35a

Modified Files
--------------
src/backend/utils/adt/selfuncs.c | 19 ++++++++++++++-----
src/test/regress/expected/stats_ext.out | 12 ++++++++++++
src/test/regress/sql/stats_ext.sql | 4 ++++
3 files changed, 30 insertions(+), 5 deletions(-)