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

[Andrei Elovikov via llvm-commits]

https://github.com/eas commented:

Now that https://github.com/llvm/llvm-project/pull/178736 has landed, can you pre-commit a test that would dump VPlan after the last pass before predicator, and then update its `RUN` line to dump after your newly added transformation as part of this PR? That would result in a diff that is exactly the result of this transformation.

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