... for convenient TNode access (e.g.: PhiAt, LoadField, LoadElement).

Bug: v8:9972
Change-Id: Ie6f5f204079cf7c84279d1cb015055816f301916
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1940254
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65243}