container/heap: remove one unnecessary comparison in Fix

The heap.Fix function calls both down and up.  If the element is moved
down, we don't need to call up and we could save a comparison.

(per suggestion by Radu Berinde)

Fixes #16098.

Change-Id: I83a74710e66cf0d274d8c0743338c26f89f31afe
Reviewed-on: https://go-review.googlesource.com/24273Reviewed-by: Robert Griesemer <gri@golang.org>

src/container/heap/heap.go

@@ -83,8 +83,9 @@ func Remove(h Interface, i int) interface{} {
 // but less expensive than, calling Remove(h, i) followed by a Push of the new value.
 // The complexity is O(log(n)) where n = h.Len().
 func Fix(h Interface, i int) {
-	down(h, i, h.Len())
+	if !down(h, i, h.Len()) {
 		up(h, i)
+	}
 }
 
 func up(h Interface, j int) {
@@ -98,7 +99,8 @@ func up(h Interface, j int) {
 	}
 }
 
-func down(h Interface, i, n int) {
+func down(h Interface, i0, n int) bool {
+	i := i0
 	for {
 		j1 := 2*i + 1
 		if j1 >= n || j1 < 0 { // j1 < 0 after int overflow
@@ -114,4 +116,5 @@ func down(h Interface, i, n int) {
 		h.Swap(i, j)
 		i = j
 	}
+	return i > i0
 }