This adds a fast path that avoids the runtime transition for JSArray
source arguments with {packed,holey} {smi,double} elements kinds.

The fast path currently calls straight into C and copies there using
elements accessor logic.

Local tests show a 4x speedup when copying from 1-element JSArrays.
As the source array becomes larger, the time spent copying elements
begins to dominate.

Bug: v8:3590
Change-Id: I05ebe54d7b255d0a76ad46ac11ce7cfd516b8ac8
Reviewed-on: https://chromium-review.googlesource.com/789010
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49634}