Use the A{,G}HI instructions where possible (4 bytes instead of 6 bytes
for A{,G}FI). Also, use 32-bit operations where appropriate for
multiplication.

Change-Id: I4041781cda26be52b54e4804a9e71552310762d0
Reviewed-on: https://go-review.googlesource.com/29733
Run-TryBot: Michael Munday <munday@ca.ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bill O'Farrell <billotosyr@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>