This is a reland of fac6f63e, after
adding initialization of unused element slots.

Original change's description:
> Use CopyElements (which uses memcpy) to copy FixedDoubleArray.
>
> This improves the performance of ExtractFixedArray and
> CloneFastJSArray for double arrays, which in turn improve the
> performance of cloning double arrays with slice() or spreading.
>
> This, however, does not improve performance of spreading holey
> double arrays, because spreading needs extra work to convert
> holes to undefined.
>
> Bug: v8:7980
> Change-Id: Ib8aed74abbb0b06982a3b754e134fa415cb7de2d
> Reviewed-on: https://chromium-review.googlesource.com/c/1280308
> Reviewed-by: Michael Stanton <mvstanton@chromium.org>
> Reviewed-by: Georg Neis <neis@chromium.org>
> Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
> Commit-Queue: Hai Dang <dhai@google.com>
> Cr-Commit-Position: refs/heads/master@{#56680}

Bug: v8:7980
Change-Id: I899af60c061b9cd6eb619c247c5fc515b92e9fd7
Reviewed-on: https://chromium-review.googlesource.com/c/1382735
Commit-Queue: Georg Neis <neis@chromium.org>
Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58333}