The scheduler could schedule unreachable nodes on two basic blocks that
later merge. Update DCHECK in graph-assembler's basic block updater to
only check for the self-containedness of unreachable basic blocks
removed from the schedule after all the blocks have been re-written to
allow for this case.

BUG=chromium:1079446,v8:9684

Change-Id: I91899dbf389e4425542dbd2b1ca95c3f6ad79c05
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2196354Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67812}