One fundamental assumption of the wasm code GC is that code becomes
"potentially dead" at most once; if the ref counts drops to zero later,
it should be freed for real.
In the current implementation, it happens that code becomes potentially
dead, then becomes dead for real (it's removed from the set of
potentially dead code), and then we remove the last reference. At that
point, we re-add the code to the potentially dead code, considering it
for garbage collection again. This can lead to an endless loop.

This CL fixes that by remembering which code was already detected as
dead, and does not consider this code for another GC.
This requires freeing code via the {WasmEngine} such that the set of
dead code can be cleaned up.

R=mstarzinger@chromium.org

Bug: v8:8217
Change-Id: If6a95a7918db2ad82edfad5447c536593243db7d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1585845Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61073}