This flag will force the usage of the mid-tier register allocator, which
is otherwise only used for huge functions, hence does not get much
fuzzer coverage.

R=machenbach@chromium.org

Bug: v8:12330
Change-Id: I30b602b54adf858a2a3aaf73d8828875c1f6df7a
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3234492Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#77500}