This CL also unifies CopyWords() and CopyBytes() implementations.

Bug: v8:7703
Change-Id: I0b2e2f35c0c651e46231c4e4286c705634dce02b
Reviewed-on: https://chromium-review.googlesource.com/c/1491602Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59912}