container/heap: optimization when selecting smaller child

In down(), if two children are equal, we can choose either one.
Inspired by https://codereview.appspot.com/6613064/

Change-Id: Iaad4ca5e2f5111bf3abb87f606584e7d274c620b
Reviewed-on: https://go-review.googlesource.com/38612
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

src/container/heap/heap.go

@@ -107,7 +107,7 @@ func down(h Interface, i0, n int) bool {
 			break
 		}
 		j := j1 // left child
-		if j2 := j1 + 1; j2 < n && !h.Less(j1, j2) {
+		if j2 := j1 + 1; j2 < n && h.Less(j2, j1) {
 			j = j2 // = 2*i + 2  // right child
 		}
 		if !h.Less(j, i) {