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

[Luke Lau via llvm-commits]

lukel97 wrote:

> @lukel97 Please land this patch before #173265. I guess we will get regression if we land #173265 before this patch.

Oh, I was actually waiting for #173265 to land first to see if we would need to generalize it to the IDom. I'll land this first though then, and if we need to generalize it then I guess you can just do that in #173265

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