This CL fixes some issues with GC.
1) It removes dead code from the set of potentially dead code to avoid
   considering the same code for GC again and again.
2) It resets the {new_potentially_dead_code_size_} counter to avoid
   triggering too many GCs.
3) When code becomes dead after GC, do not unconditionally free it; just
   decrement its ref count (there might still be {WasmCodeRefScope}s
   holding the code alive).
4) Update the comment of the ref count to be more accurate.

R=titzer@chromium.org

Bug: v8:8217
Change-Id: I28e5a1fed74411b8473bb66ddbad3ffe7643f266
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1574518
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60949}