This fixes the bug that cast<A>(...) could be interpreted as
cast<B>(...) if B is a subtype of A.

Bug: v8:7793

Change-Id: Ia03ce832f8c14ced09114d41c935be06d4629d99
Reviewed-on: https://chromium-review.googlesource.com/1075890
Commit-Queue: Tobias Tebbi <tebbi@chromium.org>
Reviewed-by: Michael Stanton <mvstanton@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53397}