Port 88062a2c

Original Commit Message:

    Implement in-place weak reference handling in GC.

    Turn FeedbackVector::optimized_code_or_smi into an in-place weak reference (this
    is the only in-place weak reference at this point).

    (See bug for design doc.)

R=marja@chromium.org, joransiu@ca.ibm.com, michael_dawson@ca.ibm.com
BUG=v8:7308
LOG=N

Change-Id: I00c6aa7c08524b7769d3428d0c18ce334f35a722
Reviewed-on: https://chromium-review.googlesource.com/949368Reviewed-by: Joran Siu <joransiu@ca.ibm.com>
Commit-Queue: Junliang Yan <jyan@ca.ibm.com>
Cr-Commit-Position: refs/heads/master@{#51747}