This allows us to just call this method without returning dummy
{nullptr} values or even calling {graph()->NewNode()} afterwards.

R=ahaas@chromium.org
BUG=v8:6474

Change-Id: I6c03eb639dfb635ce791e7d3a731d9dfdf7800a8
Reviewed-on: https://chromium-review.googlesource.com/529308
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45840}