A frame state's outer frame state input can be kDeadValue. A DCHECK
did not take that into account. In release builds there was no issue
because we branch on the opcode anyways.

While fixing this bug, I'm strengthening the FrameState class such that
a FrameState node must have a kFrameState operator. I'm also
- changing the result type of outer_frame_state() from FrameState to
  Node* since it may in fact not be a kFrameState;
- removing has_outer_frame_state() because I find it unintuitive to
  have outer_frame_state() return non-NULL even when
  has_outer_frame_state() would return true.

Bug: chromium:1224758
Change-Id: I8ebed75c62e31f7eef71e2941fd18869d8a56af3
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3001356Reviewed-by: Nico Hartmann <nicohartmann@chromium.org>
Commit-Queue: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#75553}