For synchronous compilation, the foreground thread takes part in
compilation. It should only compile Liftoff functions though, otherwise
it might unnecessarily delay initial compilation by executing expensive
TurboFan units.

Drive-by: Minor refactoring to avoid else after return.

R=mstarzinger@chromium.org

Bug: v8:8916
Change-Id: Ibf994a9a48417943652f13096419afd73cb59f85
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1564192
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60800}