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/1280308Reviewed-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}