pgsql: Fix assertions with RI triggers in heap_update and heap_delete.

Fix assertions with RI triggers in heap_update and heap_delete.

If the tuple being updated is not visible to the crosscheck snapshot,
we return TM_Updated but the assertions would not hold in that case.
Move them to before the cross-check.

Fixes bug #17893. Backpatch to all supported versions.

Author: Alexander Lakhin
Backpatch-through: 12
Discussion: https://www.postgresql.org/message-id/17893-35847009eec517b5%40postgresql.org

Branch
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REL_15_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/2873fbfe0d6500a45a316d2d42414a432a96e9f1

Modified Files
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src/backend/access/heap/heapam.c | 41 +++++++++++++++++------------
src/include/access/tableam.h | 4 +--
src/test/isolation/expected/fk-snapshot.out | 22 ++++++++++++++++
src/test/isolation/specs/fk-snapshot.spec | 17 +++++++++++-
4 files changed, 64 insertions(+), 20 deletions(-)