Fix mjsunit's assertOptimized and assertUnoptimized.

This was broken some time ago by a refactor.

R=ulan@chromium.org

Review URL: https://codereview.chromium.org/349423003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21995 ce2b1a6d-e550-0410-aec6-3dcde31c8c00