This fixes the issues 
https://bugs.chromium.org/p/chromium/issues/detail?id=772873 
and https://bugs.chromium.org/p/chromium/issues/detail?id=772872.

One problem was that mutating an effect node into Unreachable confused 
the LoadElimination sidetables, so I just always create a new node now.

The other problem was that UpdateBlockControl() was executed after 
UpdateEffectPhi() in the lazy case. This reverted the update to the Merge input.
So now I make sure that UpdateEffectPhi() is always executed last.

This is a reland of 6ddb5e7d
Original change's description:
> Reland^2 "[turbofan] eagerly prune None types and deadness from the graph"
> 
> 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/702264
> Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
> Commit-Queue: Tobias Tebbi <tebbi@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#48366}

Bug: chromium:741225
Change-Id: I4054a694d2521c2e1f0c4a3ad0f3cf100b5c536f
Reviewed-on: https://chromium-review.googlesource.com/709214
Commit-Queue: Tobias Tebbi <tebbi@chromium.org>
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48469}