No need to sign extend input to Neg8 and Neg16.

Change-Id: I7896c83c9cdf84a34098582351a4aabf61cd6fdd
Reviewed-on: https://go-review.googlesource.com/102675
Run-TryBot: Michael Munday <mike.munday@ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>