By using memmove for SMI elements we get a roughly 3x speedup over the slower
iterative copying with write barriers.

Bug: chromium:835558
Change-Id: I73da07a1648a3495ff78212ffa1ed949d205a7d2
Reviewed-on: https://chromium-review.googlesource.com/1028236Reviewed-by: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52792}