Instead of holding four ZoneLists, which always have the same length
anyway, just store all information in one struct, being held in one
ZoneList.

There is probably more optimization potential in the generation and
encoding of safepoint tables; this is a first step to reduce its
overhead.

R=mstarzinger@chromium.org

Bug: v8:7109
Change-Id: Ib4f752958c8d5713b46324e76003cd815e5ffd2d
Reviewed-on: https://chromium-review.googlesource.com/817197
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50006}