In addition to using the {Dead} node to prune dead control nodes and nodes that 
depend on them, we introduce a {DeadValue} node representing an impossible value 
that can occur at any position in the graph. The extended {DeadCodeElimination}
prunes {DeadValue} and its uses, inserting a crashing {Unreachable} node into
the effect chain when possible. The remaining uses of {DeadValue} are handled
in {EffectControlLinearizer}, where we always have access to the effect chain.
In addition to explicitly introduced {DeadValue} nodes, we consider any value use
of a node with type {None} as dead.

Bug: chromium:741225
Change-Id: Icc4b636d1d018c452ba1a2fa7cd3e00e522f1655
Reviewed-on: https://chromium-review.googlesource.com/641250
Commit-Queue: Tobias Tebbi <tebbi@chromium.org>
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48208}