INC/DEC produces slightly faster and smaller code.

Change-Id: I329d9bdb01b90041be45e053d9df640818bf0c2d
Reviewed-on: https://go-review.googlesource.com/19238
Run-TryBot: Ilya Tocar <ilya.tocar@intel.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>