This reduces the number of {BackgroundCompileScope}s per compilation
unit from 2 to 1. This reduces the number of concurrent accesses to the
{SharedMutex}.

R=ahaas@chromium.org

Bug: v8:8916
Cq-Include-Trybots: luci.v8.try:v8_linux64_tsan_rel
Change-Id: I872683101dfc9361aa7054ea67f16e3f3f2b82b1
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1495989
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60034}