All wasm code has an initial ref count of 1, in the expectation that it
will be added to the code table. When the code is removed from that
table, the ref count will be decremented.
Stepping code (and also other code under special circumstances) will not
be added to the code table though. Hence the ref count will never be
decremented below 1, and the code will never be garbage-collected.

This CL fixes this, by decrementing the ref count if the code is not
added to the code table.
Note that the code will only be collected if no isolate is currently
using it, so it won't be collected while still in use for stepping.

R=thibaudm@chromium.org

Bug: chromium:1168564
Change-Id: I3047753591cbc52689ca019e9548ec58c237b835
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2649040
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Thibaud Michaud <thibaudm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72354}