We only want to serialize TurboFan code, because Liftoff code could
contain breakpoints, and we start thinking about embedding other
non-relocatable constants.
Thus, wait until top-tier compilation finished before triggering
serialization.

A follow-up CL will make serialization fail if any Liftoff code is
encountered.

R=ahaas@chromium.org

Bug: v8:10777
Change-Id: I73d6c2d868545fcd4069a8cf9850ca7fca375ecb
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2349293Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69332}