Remove nbtree BTreeTupleSetAltHeapTID() function.

Since heap TID is supposed to be just another key attribute to the
implementation, it doesn't make much sense to have separate
BTreeTupleSetNAtts() and BTreeTupleSetAltHeapTID() functions.  Merge the
two functions together.  This slightly simplifies _bt_truncate().

src/backend/access/nbtree/nbtinsert.c

@@ -2413,7 +2413,7 @@ _bt_newroot(Relation rel, Buffer lbuf, Buffer rbuf)
 	left_item = (IndexTuple) palloc(left_item_sz);
 	left_item->t_info = left_item_sz;
 	BTreeTupleSetDownLink(left_item, lbkno);
-	BTreeTupleSetNAtts(left_item, 0);
+	BTreeTupleSetNAtts(left_item, 0, false);
 
 	/*
 	 * Create downlink item for right page.  The key for it is obtained from
@@ -2571,7 +2571,7 @@ _bt_pgaddtup(Page page,
 	{
 		trunctuple = *itup;
 		trunctuple.t_info = sizeof(IndexTupleData);
-		BTreeTupleSetNAtts(&trunctuple, 0);
+		BTreeTupleSetNAtts(&trunctuple, 0, false);
 		itup = &trunctuple;
 		itemsize = sizeof(IndexTupleData);
 	}

src/backend/access/nbtree/nbtsort.c

@@ -771,7 +771,7 @@ _bt_sortaddtup(Page page,
 	{
 		trunctuple = *itup;
 		trunctuple.t_info = sizeof(IndexTupleData);
-		BTreeTupleSetNAtts(&trunctuple, 0);
+		BTreeTupleSetNAtts(&trunctuple, 0, false);
 		itup = &trunctuple;
 		itemsize = sizeof(IndexTupleData);
 	}
@@ -1045,7 +1045,7 @@ _bt_buildadd(BTWriteState *wstate, BTPageState *state, IndexTuple itup,
 		Assert(state->btps_lowkey == NULL);
 		state->btps_lowkey = palloc0(sizeof(IndexTupleData));
 		state->btps_lowkey->t_info = sizeof(IndexTupleData);
-		BTreeTupleSetNAtts(state->btps_lowkey, 0);
+		BTreeTupleSetNAtts(state->btps_lowkey, 0, false);
 	}
 
 	/*

src/backend/access/nbtree/nbtutils.c

@@ -2239,7 +2239,7 @@ _bt_truncate(Relation rel, IndexTuple lastleft, IndexTuple firstright,
 	 */
 	if (keepnatts <= nkeyatts)
 	{
-		BTreeTupleSetNAtts(pivot, keepnatts);
+		BTreeTupleSetNAtts(pivot, keepnatts, false);
 		return pivot;
 	}
 
@@ -2262,11 +2262,13 @@ _bt_truncate(Relation rel, IndexTuple lastleft, IndexTuple firstright,
 	/* Cannot leak memory here */
 	pfree(pivot);
 
-	/* Store heap TID in enlarged pivot tuple */
+	/*
+	 * Store all of firstright's key attribute values plus a tiebreaker heap
+	 * TID value in enlarged pivot tuple
+	 */
 	tidpivot->t_info &= ~INDEX_SIZE_MASK;
 	tidpivot->t_info |= newsize;
-	BTreeTupleSetNAtts(tidpivot, nkeyatts);
-	BTreeTupleSetAltHeapTID(tidpivot);
+	BTreeTupleSetNAtts(tidpivot, nkeyatts, true);
 	pivotheaptid = BTreeTupleGetHeapTID(tidpivot);
 
 	/*

src/include/access/nbtree.h

@@ -460,34 +460,30 @@ BTreeTupleSetDownLink(IndexTuple pivot, BlockNumber blkno)
 	)
 
 /*
- * Set number of attributes in tuple, making it into a pivot tuple
+ * Set number of key attributes in tuple.
+ *
+ * The heap TID tiebreaker attribute bit may also be set here, indicating that
+ * a heap TID value will be stored at the end of the tuple (i.e. using the
+ * special pivot tuple representation).
  */
 static inline void
-BTreeTupleSetNAtts(IndexTuple itup, int natts)
+BTreeTupleSetNAtts(IndexTuple itup, uint16 nkeyatts, bool heaptid)
 {
-	Assert(natts <= INDEX_MAX_KEYS);
+	Assert(nkeyatts <= INDEX_MAX_KEYS);
+	Assert((nkeyatts & BT_RESERVED_OFFSET_MASK) == 0);
+	Assert(!heaptid || nkeyatts > 0);
+	Assert(!BTreeTupleIsPivot(itup) || nkeyatts == 0);
 
 	itup->t_info |= INDEX_ALT_TID_MASK;
-	/* BT_IS_POSTING bit may be unset -- tuple always becomes a pivot tuple */
-	ItemPointerSetOffsetNumber(&itup->t_tid, natts);
+
+	if (heaptid)
+		nkeyatts |= BT_PIVOT_HEAP_TID_ATTR;
+
+	/* BT_IS_POSTING bit is deliberately unset here */
+	ItemPointerSetOffsetNumber(&itup->t_tid, nkeyatts);
 	Assert(BTreeTupleIsPivot(itup));
 }
 
-/*
- * Set the bit indicating heap TID attribute present in pivot tuple
- */
-static inline void
-BTreeTupleSetAltHeapTID(IndexTuple pivot)
-{
-	OffsetNumber existing;
-
-	Assert(BTreeTupleIsPivot(pivot));
-
-	existing = ItemPointerGetOffsetNumberNoCheck(&pivot->t_tid);
-	ItemPointerSetOffsetNumber(&pivot->t_tid,
-							   existing | BT_PIVOT_HEAP_TID_ATTR);
-}
-
 /*
  * Get/set leaf page's "top parent" link from its high key.  Used during page
  * deletion.
@@ -505,7 +501,7 @@ static inline void
 BTreeTupleSetTopParent(IndexTuple leafhikey, BlockNumber blkno)
 {
 	ItemPointerSetBlockNumber(&leafhikey->t_tid, blkno);
-	BTreeTupleSetNAtts(leafhikey, 0);
+	BTreeTupleSetNAtts(leafhikey, 0, false);
 }
 
 /*