Safepoint maps record all spill slots that contain a tagged value. The
introduction of multi-value return changed the stack frame layout though
and the calculation of spill slots has not been adjusted accordingly.
This CL adjusts the creation of safepoints now to work for multi-value
returns as well.

R=neis@chromium.org

Bug: v8:11206
Change-Id: Id623dbc28b976dcf625ac78738e03e642fafbb36
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2569762
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71591}