Very similar to the recent ArrayPrototypeForEach port, this moves the
Reduce reduction, which previously operated directly on the graph, to
graph assembler.

Nothing too interesting here, but it's becoming clearer that we will
need more flexible Branch (multiple merge values) and If (a `break`
mechanism) handling in the future.

Bug: v8:9972
Change-Id: Ic48c85305ba721a9a43c67f7ad13c60da310487e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1934329
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Michael Stanton <mvstanton@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65233}