Revert "nbtree: Remove useless row compare arg."

This reverts commit 54c6ea8c81.

Further analysis has shown that the forcenonrequired row compare
behavior is in fact necessary, despite the new restrictions on
RowCompares imposed by _bt_set_startikey following commit 5f4d98d4.

Discussion: https://postgr.es/m/CAH2-Wzm3bKcz3TbHGem3_+SinEyG=VZVPbApQghp7YiZj+MM3g@mail.gmail.com

src/backend/access/nbtree/nbtutils.c

@@ -63,7 +63,7 @@ static bool _bt_check_compare(IndexScanDesc scan, ScanDirection dir,
 							  bool *continuescan, int *ikey);
 static bool _bt_check_rowcompare(ScanKey skey,
 								 IndexTuple tuple, int tupnatts, TupleDesc tupdesc,
-								 ScanDirection dir, bool *continuescan);
+								 ScanDirection dir, bool forcenonrequired, bool *continuescan);
 static void _bt_checkkeys_look_ahead(IndexScanDesc scan, BTReadPageState *pstate,
 									 int tupnatts, TupleDesc tupdesc);
 static int	_bt_keep_natts(Relation rel, IndexTuple lastleft,
@@ -2902,10 +2902,8 @@ _bt_check_compare(IndexScanDesc scan, ScanDirection dir,
 		/* row-comparison keys need special processing */
 		if (key->sk_flags & SK_ROW_HEADER)
 		{
-			Assert(!forcenonrequired);	/* forbidden by _bt_set_startikey */
-
 			if (_bt_check_rowcompare(key, tuple, tupnatts, tupdesc, dir,
-									 continuescan))
+									 forcenonrequired, continuescan))
 				continue;
 			return false;
 		}
@@ -3062,7 +3060,8 @@ _bt_check_compare(IndexScanDesc scan, ScanDirection dir,
  */
 static bool
 _bt_check_rowcompare(ScanKey skey, IndexTuple tuple, int tupnatts,
-					 TupleDesc tupdesc, ScanDirection dir, bool *continuescan)
+					 TupleDesc tupdesc, ScanDirection dir,
+					 bool forcenonrequired, bool *continuescan)
 {
 	ScanKey		subkey = (ScanKey) DatumGetPointer(skey->sk_argument);
 	int32		cmpresult = 0;
@@ -3102,7 +3101,11 @@ _bt_check_rowcompare(ScanKey skey, IndexTuple tuple, int tupnatts,
 
 		if (isNull)
 		{
-			if (subkey->sk_flags & SK_BT_NULLS_FIRST)
+			if (forcenonrequired)
+			{
+				/* treating scan's keys as non-required */
+			}
+			else if (subkey->sk_flags & SK_BT_NULLS_FIRST)
 			{
 				/*
 				 * Since NULLs are sorted before non-NULLs, we know we have
@@ -3156,8 +3159,12 @@ _bt_check_rowcompare(ScanKey skey, IndexTuple tuple, int tupnatts,
 			 */
 			Assert(subkey != (ScanKey) DatumGetPointer(skey->sk_argument));
 			subkey--;
-			if ((subkey->sk_flags & SK_BT_REQFWD) &&
-				ScanDirectionIsForward(dir))
+			if (forcenonrequired)
+			{
+				/* treating scan's keys as non-required */
+			}
+			else if ((subkey->sk_flags & SK_BT_REQFWD) &&
+					 ScanDirectionIsForward(dir))
 				*continuescan = false;
 			else if ((subkey->sk_flags & SK_BT_REQBKWD) &&
 					 ScanDirectionIsBackward(dir))
@@ -3209,7 +3216,7 @@ _bt_check_rowcompare(ScanKey skey, IndexTuple tuple, int tupnatts,
 			break;
 	}
 
-	if (!result)
+	if (!result && !forcenonrequired)
 	{
 		/*
 		 * Tuple fails this qual.  If it's a required qual for the current