Many safepoint tables do not contain any deoptimization info and/or no
callee-saved registers. Do not emit empty fields for all entries in this
case.
This often shrinks the size of the encoded safepoint table by more than
50%.

Drive-by cleanups:
- Rename fields of the safepoint table entries to clarify their meaning
("tagged slots" instead of "bits", "tagged register indexes" instead of
 "register bits").
- Include the PC in the decoded {SafepointEntry} to make it the single
source of truth.

R=jkummerow@chromium.org

Bug: v8:12401
Change-Id: If5c24a688a434842ed3b6427f5f1f3ea9232173a
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3289173Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#78021}