Now that the optimized code hangs off the feedback vector, it is possible
to check whether a function has optimized code available every time it's
called in the interpreter entry trampoline. If optimized code exists, the
interpreter entry trampoline 'self-heals' the closure to point to the
optimized code and links the closure into the optimized code list.

BUG=v8:6246

Change-Id: If1bd7c555bb0551bfe04b36baa6bcf949604717e
Reviewed-on: https://chromium-review.googlesource.com/488026Reviewed-by: Michael Stanton <mvstanton@chromium.org>
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45103}