Move BufferGetBlockNumber() out of heap_page_is_all_visible()'s inner loop.

In some workloads BufferGetBlockNumber() shows up in profiles due to
the sheer number of calls to it (and because it causes cache
misses). The compiler can't move it out of the loop because it's a
full extern function call...

src/backend/commands/vacuumlazy.c

@@ -1754,6 +1754,7 @@ static bool
 heap_page_is_all_visible(Relation rel, Buffer buf, TransactionId *visibility_cutoff_xid)
 {
 	Page		page = BufferGetPage(buf);
+	BlockNumber	blockno = BufferGetBlockNumber(buf);
 	OffsetNumber offnum,
 				maxoff;
 	bool		all_visible = true;
@@ -1778,7 +1779,7 @@ heap_page_is_all_visible(Relation rel, Buffer buf, TransactionId *visibility_cut
 		if (!ItemIdIsUsed(itemid) || ItemIdIsRedirected(itemid))
 			continue;
 
-		ItemPointerSet(&(tuple.t_self), BufferGetBlockNumber(buf), offnum);
+		ItemPointerSet(&(tuple.t_self), blockno, offnum);
 
 		/*
 		 * Dead line pointers can have index pointers pointing to them. So