pg_dump: ALTER INDEX SET STATISTICS missing for index-backed constraints

Hello hackers,

I noticed this while reviewing the pg_get_tale_ddl patch and looking
for inconsistencies compared to pg_dump.

It seems like pg_dump emits SET STATISTICS for most cases since 2018
(e4fca461), but it was missed in this case:

CREATE TABLE t (a int, b int);
ALTER TABLE t ADD CONSTRAINT name EXCLUDE USING gist ((a+b) WITH =);
ALTER INDEX name ALTER COLUMN 1 SET STATISTICS 900;

The last statement was missing from the pg_dump output.

Attached patch fixes it and adds a test case.