This makes it clear that the constructors are to be used for a basic block that starts a loop or a basic block that starts an exception handler.

Bug: v8:7700
Change-Id: Ic58dd80f223e45b4e5cf5da6e78e989ba18d55a3
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3870916
Auto-Submit: Victor Gomes <victorgomes@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/main@{#82944}