Only compile concurrently if enabled, in %OptimizeFunctionOnNextCall.

R=ulan@chromium.org
BUG=326474
LOG=N

Review URL: https://codereview.chromium.org/106403007

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@18403 ce2b1a6d-e550-0410-aec6-3dcde31c8c00