MGHI (16-bit signed immediate) is now used where possible for both
MULLW and MULLD. MGHI is 2-bytes shorter than MSGFI.

Change-Id: I5d0648934f28b3403b1126913fd703d8f62b9e9f
Reviewed-on: https://go-review.googlesource.com/22398
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>