prepare for ptr compression

Change-Id: Ia459ad18a30fbfd8e51bd99735f0d63644d30b8f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1831788Reviewed-by: Joran Siu <joransiu@ca.ibm.com>
Reviewed-by: Junliang Yan <jyan@ca.ibm.com>
Commit-Queue: Junliang Yan <jyan@ca.ibm.com>
Cr-Commit-Position: refs/heads/master@{#64051}