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>