Actually apply the folding on allocation.

Bug: chromium:1056170
Change-Id: Ief25fac49c5caff40579fd44249bba0ae82f9689
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2756536
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Auto-Submit: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Omer Katz <omerkatz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73377}