Local testing shows that this switch for small counts (up to size 16)
is significantly faster than the default {std::copy_n} (by up to 20%,
e.g. for the "join-int" js perf test). It's also faster than just a
loop covering all sizes up to 16.

R=leszeks@chromium.org
CC=jkummerow@chromium.org

Bug: chromium:1006157
Change-Id: I4d179f064704261fa18f453c23c04ee0b351e942
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1864831Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64397}