We currently only use the predictable platform if --verify-predictable
is passed, which is confusing and not following the comment on the
--predictable flag ("enable predictable mode").

This CL fixes that and makes --verify-predictable imply --predictable to
also allow to only pass --verify-predictable.

R=ahaas@chromium.org
CC=mlippautz@chromium.org

Bug: v8:11879
Change-Id: Ifb9683ddc4fab374ce519169533c90244175bb48
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3094010Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#76305}