[llvm] [LV] Move predication, early exit & region handling to VPlan0 (NFCI) (PR #185305)

[Luke Lau via llvm-commits]

lukel97 wrote:

> I can see how early exits and region handling are VF-independent, but tail folding and predication aren't (at least in theory, even if they are currently). Do the latter really need to be moved?

What parts of tail folding and predication would be VF specific? AFAIU VF-independent in this context means it doesn't use `LoopVectorizationPlanner::getDecisionAndClampRange`.

If we're talking about cost modelling tail folded vs non-tail folded VFs, I would have thought we would have just created separate VPlans to begin with anyway. 

https://github.com/llvm/llvm-project/pull/185305