We only need to materialize the existing output register for a given
register transfer if it is in a different equivalence set, otherwise we
already have the value we want in the output register.

BUG=v8:4280

Change-Id: Ic4966590ac10445180aff353940d2c93e6a818aa
Reviewed-on: https://chromium-review.googlesource.com/493168Reviewed-by: Mythri Alle <mythria@chromium.org>
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45051}