This CL uses MemCopy in CopyElements, because we think that the input
FixedArrays can never overlap. This CL also adds a DCHECK to ensure
this. If our assumption is wrong, it will be interesting to learn why.

Change-Id: Ib8258b934f43b82b1281481c592762e43b35938f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1549154
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Hannes Payer <hpayer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60589}