The same change on the {CopyChars} function gave an unexpected 10-20%
speedup on microbenchmarks across platforms (ia32, x64, Atom_x64).
This CL explores whether a similar change generally speeds up {MemCopy}
and {MemMove} (only on x64 for now). If this is the case, we might be
able to carefully extend the same pattern to other platforms and remove
custom assembly implementations without too much regression.

R=leszeks@chromium.org

Bug: v8:9810
Change-Id: Ib9674807b67cd2a463680b97a91ae1b41c3be65b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1871607Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64498}