Although the result was unused, these functions used to return a (often
random) Node* to satisfy old restrictions of graph-builder-interface.
Now that these restrictions are lifted, we can type them properly as
{void}.

Change-Id: I914024240f3005bc8a8636ac33ed4594f5ae5988
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2767218
Commit-Queue: Manos Koukoutos <manoskouk@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73544}