Does not pass toolstash, but only because it causes ATYPE instructions
to be emitted in a different order, and it avoids emitting type
metadata for unused variables.

Change-Id: I3ec8f66a40b5af9213e0d6e852b267a8dd995838
Reviewed-on: https://go-review.googlesource.com/30217
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>