Port 73d21080

Original Commit Message:

    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.

R=rmcilroy@chromium.org, joransiu@ca.ibm.com, jyan@ca.ibm.com, michael_dawson@ca.ibm.com
BUG=v8:6246
LOG=N

Review-Url: https://codereview.chromium.org/2897483002
Cr-Commit-Position: refs/heads/master@{#45410}