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 <knizhnik@garret.ru>
Reviewed-by: Peter Geoghegan <pg@bowt.ie>
Reviewed-by: Mihail Nikalayeu <mihailnikalayeu@gmail.com>
Discussion: https://www.postgresql.org/message-id/33e39552-6a2a-46f3-8b34-3f9f8004451f@garret.ru
Backpatch-through: 14

contrib/amcheck/verify_nbtree.c

@@ -2047,7 +2047,7 @@ bt_child_highkey_check(BtreeCheckState *state,
 		 * If we visit page with high key, check that it is equal to the
 		 * target key next to corresponding downlink.
 		 */
-		if (!rightsplit && !P_RIGHTMOST(opaque))
+		if (!rightsplit && !P_RIGHTMOST(opaque) && !P_ISHALFDEAD(opaque))
 		{
 			BTPageOpaque topaque;
 			IndexTuple	highkey;