[llvm] [VPlan] Model branch cond to enter scalar epilogue in VPlan. (PR #92651)

[Florian Hahn via llvm-commits]

fhahn wrote:

Split off the VPIRWrapperBlock changes to https://github.com/llvm/llvm-project/pull/93398 to use it for the pre-preheader, which required a bit of generalization.

https://github.com/llvm/llvm-project/pull/92651