Turbofan's instruction scheduling is currently only enabled for
mksnapshot and has thus little test coverage. This CL introduces two
new test variants, "instruction_scheduling" and
"stress_instruction_scheduling", and activates them on a selection of
bots.

Change-Id: I5917fc781e289377c58f584c770c91e31765b2de
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1899778
Auto-Submit: Georg Neis <neis@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Michael Stanton <mvstanton@chromium.org>
Commit-Queue: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64807}