We plan to trim the size of safepoint entries, to avoid emitting
completely empty entries, and also saving some bytes by removing
trailing zeros.
This CL prepares that by removing the assumption that the safepoint
entry contains one bit per stack slot. Instead, we just use all bits
that are there in the safepoint entry when iterating compiled frames.

R=jkummerow@chromium.org

Bug: v8:11630
Change-Id: Ib335a34da92a08e28fe84fb74f50c8535c2f4c41
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2831484Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74031}