It's possible for a one-byte string to wrap a two-byte string. For
example:

      CONS_ONE_BYTE_STRING_TYPE
wraps THIN_STRING_TYPE
wraps EXTERNAL_INTERNALIZED_STRING_TYPE

We thus have to validate the extracted instance type when relying on
the string being one-byte.

Bug: chromium:1088179
Change-Id: Id0f996761a3ae2f1233e12c95e663e77d4a5ebf9
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2226752Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68124}