If the same VariableProxy is added to a ThreadedList twice, it will
introduce a cycle which would cause a DCHECK failure on the next Add.
This makes it DCHECK as soon as an element is added that already has a
"next" value set.

Change-Id: I720ff6100d8d43e264726039ddaf97c205a5062f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1872391
Auto-Submit: Dan Elphick <delphick@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64452}