If a thin string can be dereferenced for StringConcat we still need to check
whether the dereferenced string is a sequential string itself (it could be
an external string).

BUG=v8:6243

Change-Id: I146541512525726f092580512c0b5f02d33685a7
Reviewed-on: https://chromium-review.googlesource.com/558994
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46459}