This CL allows ExtractFixedArray to know if the JSArray has only Smi
elements. In that case ExtractFixedArray will always skip write barriers
when copying the Smi-only backing store. In the case that the copying
is not possibly conflicting with the concurrent marker, CopyElements will
efficiently use memcpy for further performance.

This improves the performance of ExtractFastJSArray and CloneFastJSArray.
As a result, performance of copying Smi arrays by slice() and spreading is
improved, except for spreading Smi holey arrays.

Bug: v8:7980
Change-Id: Ie39f8abf3b5039cc12a8ca7ece81352375e0e9da
Reviewed-on: https://chromium-review.googlesource.com/c/1286340Reviewed-by: Georg Neis <neis@chromium.org>
Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Reviewed-by: Michael Stanton <mvstanton@chromium.org>
Commit-Queue: Hai Dang <dhai@google.com>
Cr-Commit-Position: refs/heads/master@{#56989}