This reverts commit 453cf219.

Reason for revert: Multiple compile errors, e.g. https://ci.chromium.org/p/v8/builders/ci/V8%20Linux%20-%20debug%20builder/45301

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=jgruber@chromium.org,tebbi@chromium.org

Change-Id: Ic81321960da36e8ddcdc8e0072b2e9cd41206478
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: v8:8888
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2390646Reviewed-by: Clemens Backes <clemensb@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69682}