Don't try to fix eliminated nbtree array scan keys.

Preprocessing for nbtree index scans allowed array "input" scan keys
already marked eliminated during array-specific preprocessing to be
"fixed up" during preprocessing proper.  This allowed eliminated scan
keys on DESC index columns to spurious have their strategy commuted,
causing assertion failures.

To fix, teach _bt_fix_scankey_strategy to ignore these scan keys.  This
brings it in line with its only caller, _bt_preprocess_keys.

Oversight in commit 5bf748b, which enhanced nbtree ScalarArrayOp
execution.

Reported-By: Donghang Lin <donghanglin@gmail.com>
Discussion: https://postgr.es/m/CAA=D8a2sHK6CAzZ=0CeafC-Y-MFXbYxnRSHvZTi=+JHu6kAa8Q@mail.gmail.com

Author: petergeoghegan

src/backend/access/nbtree/nbtutils.c

@@ -3394,6 +3394,13 @@ _bt_fix_scankey_strategy(ScanKey skey, int16 *indoption)
 		return true;
 	}
 
+	if (skey->sk_strategy == InvalidStrategy)
+	{
+		/* Already-eliminated array scan key; don't need to fix anything */
+		Assert(skey->sk_flags & SK_SEARCHARRAY);
+		return true;
+	}
+
 	/* Adjust strategy for DESC, if we didn't already */
 	if ((addflags & SK_BT_DESC) && !(skey->sk_flags & SK_BT_DESC))
 		skey->sk_strategy = BTCommuteStrategyNumber(skey->sk_strategy);