The concurrent marker visits arrays with fast SMI elements because they
have the same visitor id as arrays with tagged elements.

Visiting concurrently with memmove can be unsafe depending on memmove
implementation.

Bug: chromium:694255
Change-Id: Ic6c2cae8761e5b1b042e4274d4f90ac59f32d91f
Reviewed-on: https://chromium-review.googlesource.com/712158
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48454}