GetMaxConcurrency() needs to return a value greater than 0 when there
is work left. When the return value is 0, no more items are processed.

With Minor MC it could happen that GetMaxConcurrency() returned 0 when
there were no old-to-new-slots even though there were still items left
to process. This CL fixes this and adds a DCHECK to ensure this doesn't
happen again.

Change-Id: Ia971c232564bcb0b0d305e76371a3a8e82f46229
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2593247
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Reviewed-by: Hannes Payer <hpayer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71768}