In certain situations a phi might not be used by later code, and so
is neither spilled nor has a register allocated to it. Handle this
by removing the incorrect DCHECK.

BUG=chromium:1137979,v8:9684

Change-Id: I702dc05dba22e23dac5c1a366a770f18bac45c52
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2471998
Commit-Queue: Tobias Tebbi <tebbi@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Auto-Submit: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70536}