Currently, we call the MapRef::AsElementsKind method on an initial
map multiple times (from JSCreateLowering::ReduceJSCreateArray).
However, this does not does not play well with the heap copier/broker,
which only expectes AsElementsKind to be called on initial maps.

This CL makes sure we only call AsElementsKind once (on the initial map).

Bug: chromium:890620
Change-Id: If44421d3900abb7629ea8f789a005b8d8ebaf881
Reviewed-on: https://chromium-review.googlesource.com/1253105Reviewed-by: Georg Neis <neis@chromium.org>
Commit-Queue: Jaroslav Sevcik <jarin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56307}