For unclear reasons, cacheSpan and uncacheSpan compute the number of
elements in a span by dividing its size by the element size. This
number is simply available in the mspan structure, so just use it.

Change-Id: If2e5de6ecec39befd3324bf1da4a275ad000932f
Reviewed-on: https://go-review.googlesource.com/c/138656
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>