Not sure why it was ever disabled (early SSA work?) but it passes now.

Change-Id: I76439cacdbd286ce077f7e08c4d0663396a0cd8f
Reviewed-on: https://go-review.googlesource.com/30672Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>