It shrinks Prog type from 448 bytes down to 376 bytes on amd64.

It also makes sense, because I don't know of any modern architecture
that have instructions which can write to two destinations, none of
which is a register (even x86 doesn't have such instructions).

Change-Id: I3061f1c9ac93d79ee2b92ecb9049641d0e0f6300
Reviewed-on: https://go-review.googlesource.com/10330Reviewed-by: Aram Hăvărneanu <aram@mgk.ro>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Russ Cox <rsc@golang.org>