DeadValue was a constant node of type None. This is unsound in the
presence of re-scheduling. This CL adds a value input to DeadValue,
which preserves the dependency on the original node of type None.

Bug: chromium:796041
Change-Id: I3ac459bf661fb78c56552e8201aa18a7dbc4d182
Reviewed-on: https://chromium-review.googlesource.com/847011
Commit-Queue: Tobias Tebbi <tebbi@chromium.org>
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50340}