This makes sure the interpreter clears any stale references from the
reference stack when they are popped/dropped. Otherwise stale values
would unnecessarily increase lifetime of operand stack slots.

R=ahaas@chromium.org
BUG=v8:7581

Change-Id: I6b8be56a815327229a66ea0c97b3646ac64f6461
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1612905Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61510}