Prior to this CL, one had to artificially insert a
basic-block-terminating node after Unreachable. The common pattern was

 Unreachable();
 Goto(&some_label);  // Never reached but generates useless code.

This CL improves usability by automatically merging Unreachable nodes
to the end node, and terminating current effect/control. The updated
pattern is just

 Unreachable();

or in cases where Turboprop must maintain a schedule:

 Unreachable(&some_label);

Bug: v8:8888
Change-Id: I26a0b11b5e67252a6dc3584ae09ed06370f1eacc
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2362690
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Auto-Submit: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69531}