Also further tighten-up that calling DCHECK in BytecodeGraphBuilder,
and narrow the other caller to IsValidReferenceExpression.

Bug: v8:6092
Change-Id: I432a3d6f5991f2d1adf4f4f86e80d6ed8be5a0e8
Reviewed-on: https://chromium-review.googlesource.com/648196Reviewed-by: Mythri Alle <mythria@chromium.org>
Commit-Queue: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47833}