[llvm] [VPlan] Move tail folding out of VPlanPredicator. NFC (PR #176143)

[Andrei Elovikov via llvm-commits]

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eas wrote:

I think you don't need/want to update with NotPoison filter here only because it's actually NotMySpecificPoisonCreatedInSomeSpecificTransform. Can we just make it a generic NotPoison, update here as well and possibly do that in a separate PR?

https://github.com/llvm/llvm-project/pull/176143