container/heap: avoid up() invoke if down() success at heap.Remove()

Change-Id: I6c210e0c23ca533e1f303f88ef9dcb629a294a2a
Reviewed-on: https://go-review.googlesource.com/43472
Reviewed-by: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>

src/container/heap/heap.go

@@ -72,9 +72,10 @@ func Remove(h Interface, i int) interface{} {
 	n := h.Len() - 1
 	if n != i {
 		h.Swap(i, n)
-		down(h, i, n)
+		if !down(h, i, n) {
 			up(h, i)
 		}
+	}
 	return h.Pop()
 }