We encode the reachability/liveness in the placement. After we prepare
use counts, the kUnknown placement means that the noe is unreachable.

Bug: v8:5267
Change-Id: Iad27159508f0aefb812b6394a257055f789fbe13
Reviewed-on: https://chromium-review.googlesource.com/646247
Commit-Queue: Jaroslav Sevcik <jarin@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47795}