exp/ssa: fix breakage due to https://code.google.com/p/go/source/detail?r=ca5e5de48173
exp/ssa: fix breakage due to https://code.google.com/p/go/source/detail?r=ca5e5de48173

I don't understand why this didn't show up during my testing.

R=bradfitz
TBR=bradfitz
CC=golang-dev
https://codereview.appspot.com/7237047
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go