Fix amcheck's handling of half-dead B-tree pages

Fix amcheck's handling of half-dead B-tree pages

amcheck incorrectly reported the following error if there were any
half-dead pages in the index:

ERROR:  mismatch between parent key and child high key in index
"amchecktest_id_idx"

It's expected that a half-dead page does not have a downlink in the
parent level, so skip the test.

Reported-by: Konstantin Knizhnik <[email protected]>
Reviewed-by: Peter Geoghegan <[email protected]>
Reviewed-by: Mihail Nikalayeu <[email protected]>
Discussion: https://www.postgresql.org/message-id/33e39552-6a2a-46f3-8b34-3f9f8004451f@garret.ru
Backpatch-through: 14