This CL consolidates CopyFromPrototype and RemoveArrayHoles into a
single runtime function. It also creates two small helper functions
that are needed in both pre-processing steps.

Additionally it removes the return value from CopyFromPrototype since
it is no longer needed (it was previously used by a sort post-
processing step that no longer exists).

Bug: v8:7382
Change-Id: I7f9b00c1bc639d2118fdecef9c3b45c2cf010310
Reviewed-on: https://chromium-review.googlesource.com/1051887
Commit-Queue: Simon Zünd <szuend@google.com>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53199}