This is a reland of 453cf219

Original change's description:
> [compiler] Add a StartNode wrapper class
>
> .. to make implicit semantics of output nodes explicit.
>
> Bug: v8:8888
> Change-Id: I2ea5f5fa02f3d1f51196ea1e1e46b526dd9dc7d6
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2388117
> Commit-Queue: Jakob Gruber <jgruber@chromium.org>
> Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#69681}

Tbr: tebbi@chromium.org
Bug: v8:8888
Change-Id: I7ff3fc64c607a5289981a0762ad3a2b1de5a284d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2392241
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69690}