Drive-by: Clarify code that initializes the allocation site argument
of ArrayNArgumentsConstructor to undefined. Until now, this was done
in InternalArrayConstructor prior to calling the Impl builtin. But the
allocation site is not part of the Impl interface descriptor and
worked only by chance since we did not clobber the register. Also, the
argument is only needed for the path that calls
ArrayNArgumentsConstructor and can be omitted elsewhere. This CL pulls
initialization to just before the final call.

Bug: v8:6666
Change-Id: I24471ca0291a83dfad7af4cc11d699c8923117ec
Reviewed-on: https://chromium-review.googlesource.com/1235917
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56086}