Saves about 2k for binaries in pkg/tool/linux_amd64.
Also useful when opt runs after cse (as in 12960) which reorders
arguments for commutative operations such as Add64.

Change-Id: I49ad53afa53db9736bd35c425f4fb35fb511fd63
Reviewed-on: https://go-review.googlesource.com/19827
Run-TryBot: Alexandru Moșoi <alexandru@mosoi.ro>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Keith Randall <khr@golang.org>