FindFrameStateBefore can return a FrameState or Dead.

Bug: chromium:1232668
Change-Id: I2a21a1cd4dce9053250c1940177834ce6d2cb55b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3053582
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Georg Neis <neis@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Auto-Submit: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#75907}