Add a CHECK_FOR_INTERRUPTS() call to make heap relation verification
responsive to query cancellations.

Author: Mark Dilger <mark.dilger@enterprisedb.com>
Discussion: https://postgr.es/m/CAH2-Wzk-9RtQgb2QiuLv8j2O0j9tSFKPmmch5nWSZhguUxvbrw%40mail.gmail.com
Backpatch: 14-, where amcheck heap verification was introduced.