When mark-shared-funtion-for-tier-up is enabled, a function could be marked for
optimization, then the baseline (FCG) code is flushed by the GC. The next time
the function is executed, we shouldn't optimize the code if there isn't
baseline code.

BUG=chromium:673242

Review-Url: https://codereview.chromium.org/2575333003
Cr-Commit-Position: refs/heads/master@{#41751}