Now, the EffectControlLinearizer connects all occurrences of Unreachable to the 
graph end. This fixes issues with later phases running DeadCodeElimination and
introducing new DeadValue nodes when processing uses of Unreachable.

This is a reland of 3c4bc27f
Original change's description:
> Reland "[turbofan] eagerly prune None types and deadness from the graph"
> 
> This is a reland of e1cdda25
> Original change's description:
> > [turbofan] eagerly prune None types and deadness from the graph
> > 
> > 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}
> 
> Bug: chromium:741225
> Change-Id: I21316913dae02864f7a6d7c9269405a79f054138
> Reviewed-on: https://chromium-review.googlesource.com/692034
> Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
> Commit-Queue: Tobias Tebbi <tebbi@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#48232}

Bug: chromium:741225
Change-Id: I5702ec34856c075717162153adc765774453c45f
Reviewed-on: https://chromium-review.googlesource.com/702264Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Commit-Queue: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48366}