Loop exits are marked automatically by the loop scope mechanism in
combination with MergeState. Creating a LoopExitValue node explicitly
created an invalid graph.

Drive-by: Remove LoopExit{,Effect,Value} convenience methods.

Bug: chromium:1044878,v8:9972
Change-Id: I6fb52a8463e5352b5a5b243f800a9eda47df9d5a
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2016593
Auto-Submit: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Tobias Tebbi <tebbi@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65949}