[elements] Fix slowdown from switching to atomic accesses

I found no way to speed up the (relaxed) atomic accesses, so the only
way to get back the original performance is having a separate path for
the non-shared case.

R=ulan@chromium.org

Bug: v8:11704, chromium:1206552, chromium:1207351
Change-Id: I2ea0ecf07583dfe24f4085533491a1d5709c9ffb
Cq-Include-Trybots: luci.v8.try:v8_linux64_tsan_rel_ng
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2878750Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74533}