This fixes an issue encountered in wasm payloads, where we do not
(yet) optimize away duplicate phi definitions - phis in the same block
with the same operand list; and when move optimizations merge phi-
defining moves into the block defining the phi. If all this happens, the
register allocation validator back-propagation fails because it can't
distinguish the duplicate phis, when traversing backwards.

BUG=

Review URL: https://codereview.chromium.org/1720003002

Cr-Commit-Position: refs/heads/master@{#34207}