optimizer was merging phi lowering moves into the node defining the
phi. Addressing such cases proved to be too difficult in the existing
validator.

The new validator addresses this, by making no assumptions on phi
lowering details.

This unblocks 1824303002.

BUG=

Review URL: https://codereview.chromium.org/1855023002

Cr-Commit-Position: refs/heads/master@{#35557}