[base] Check ThreadedList integrity after Add

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}

src/base/threaded-list.h

@@ -34,6 +34,8 @@ class ThreadedListBase final : public BaseClass {
     DCHECK_NULL(*TLTraits::next(v));
     *tail_ = v;
     tail_ = TLTraits::next(v);
+    // Check that only one element was added (and that hasn't created a cycle).
+    DCHECK_NULL(*tail_);
   }
 
   void AddFront(T* v) {