This gives up on earlier attempts to interpret DeadValue as a signal of 
unreachable code. This does not work because free-floating dead value 
nodes, and even pure branch nodes that use them, can get scheduled so
early that they get reachable. Instead, we now eagerly remove branches
that use DeadValue in DeadCodeElimination and replace DeadValue inputs 
to value phi nodes with dummy values.

Reland of https://chromium-review.googlesource.com/715716

Bug: chromium:741225 chromium:776256
Change-Id: I251efd507c967d4a8882ad8fd2fd96c4185781fe
Reviewed-on: https://chromium-review.googlesource.com/727893
Commit-Queue: Tobias Tebbi <tebbi@chromium.org>
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49188}