Port 37680d65

Original Commit Message:

    Instead of having feedback vector as a subtype of FixedArray with
    reserved slots, make it a first-class variable-sized object with a
    fixed-size header. This allows us to compress counters to ints in the
    header, rather than forcing them to be Smis.

R=leszeks@chromium.org, joransiu@ca.ibm.com, jyan@ca.ibm.com, michael_dawson@ca.ibm.com
BUG=
LOG=N

Change-Id: Ia835942de292c4e4b802e34672f1e8bf8a2491c7
Reviewed-on: https://chromium-review.googlesource.com/590168Reviewed-by: Junliang Yan <jyan@ca.ibm.com>
Commit-Queue: Jaideep Bajwa <bjaideep@ca.ibm.com>
Cr-Commit-Position: refs/heads/master@{#46954}