This is an extension to LoopDeletion that came up recently in another review discussion (D93734).
The basic idea is that if SCEV can prove the backedge isn't taken, we can go ahead and get rid of the backedge (and thus the loop) while leaving the rest of the control in place. This nicely handles cases with dispatch between multiple exits and internal side effects.
I'd appreciate a careful review of the LoopInfo handling here. I tried to parallel the existing deleteDeadLoop handling, but this code has subtle invariants and stale comments. Definitely room for error.