Adds support for Speculatively lower ToPrimitiveToString to CheckString
where the type hint shows the value has always been a string.

BUG=v8:6243

Change-Id: I7f36deb8c2bc309e6d0546e099c76ac518c6be09
Reviewed-on: https://chromium-review.googlesource.com/521123
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45760}