This CL extracts the branch into a new function. Previously, the branch
is only used to copy a FixedArray to a new FixedArray. The new function
generalizes this to allow copying from a FixedDoubleArray to a
FixedArray also. This function will be useful in a follow-up CL to copy
a FixedDoubleArray with holes into a FixedArray where holes are replaced
by undefined.

Bug: chromium:881273, v8:7980

Change-Id: I8a0e5f933fc152a12d67810f4cbcfdce094d44af
Reviewed-on: https://chromium-review.googlesource.com/1230913
Commit-Queue: Hai Dang <dhai@google.com>
Reviewed-by: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56101}