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

[Mel Chen via llvm-commits]

Mel-Chen wrote:

@lukel97 Please land this patch before https://github.com/llvm/llvm-project/pull/173265. I guess we will get regression if we land https://github.com/llvm/llvm-project/pull/173265 before this patch.

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