Decommitting memory ranges is expensive, so try to merge as many of them
as possible before decommitting.

Minor drive-by: Slightly extend a GC tracing message.

R=ahaas@chromium.org

Change-Id: I91e44db1212f0c9b70b8c8fccebe8fd33ff37d9c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2375825Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69564}