The IterableToList helper builtin can return the input JSArray unchanged
if the fast-path detection decides that it doesn't need to iterate the
elements, which means we can also get a JSArray with an elements kind
that is not PACKED_ELEMENTS as a result of IterableToList.

Bug: chromium:821159, v8:7310
Change-Id: I93a886e6b7f1e1a58dd05affa46fea7501cc5a81
Reviewed-on: https://chromium-review.googlesource.com/959323Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51893}