If the input array is a JSArray with fast elements, it makes sense
to create an output array of the same ElementsKind when possible.

Bug: v8:1956
Change-Id: Ie9c937cf1751ccbbbe7cc76f40e1e1a0328ed37c
Reviewed-on: https://chromium-review.googlesource.com/730748
Commit-Queue: Michael Stanton <mvstanton@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48789}