Without processing the input, a phi node can be flagged as unused and
replaced with {Dead}, although it is used by a {DeadValue} node.

Bug: chromium:808472
Change-Id: I7446883535b34770e31e4e26e1c242eb05673a91
Reviewed-on: https://chromium-review.googlesource.com/919362
Commit-Queue: Tobias Tebbi <tebbi@chromium.org>
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51395}