Previously the object cloning fast-path had a single loop which would
initialize the object _and_ at the same time clone MutableHeapNumbers.
But since that can trigger GCs, the heap verifier was a bit sad to see
double fields holding undefined values. This was flushed out by the CL
https://chromium-review.googlesource.com/1655291, which changed the GC
timing slightly and thus made the test crash in the verifier.

So instead of the one loop, we now have a second loop that takes care
of cloning any MutableHeapNumbers. This has the advantage that the first
loop can always run without write barriers.

Bug: chromium:964748, chromium:973045, v8:7611, v8:9114, v8:9183, v8:9343
Change-Id: I724a1c1e534243ce9ecde95bf0c07ca26363b515
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1655307
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Auto-Submit: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62114}