This CL allows deopts from CheckString to disable
speculation.

Bug: v8:7127, v8:6270
Change-Id: I029caeb61c509e5eb51b169ac42596d632f7c75a
Reviewed-on: https://chromium-review.googlesource.com/831866
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50172}