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

[Luke Lau via llvm-commits]

lukel97 wrote:

I've tried out the explicit DenseMap approach in ed18c28e87494515bf398919b68fbb0a0527123d which should avoid the faux-control flow now. 

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