[llvm] [VPlan] Add VPIRWrapperBlock, use to model pre-preheader. (PR #93398)

[via llvm-commits]

https://github.com/ayalz commented:

Looks right to me, thanks!

A subclass looks better than adding a bit to VPBasicBlock, right?

Perhaps `VPIRBasicBlock` would be a good name, being both.

According to the roadmap, there are two such basic blocks: scalar.ph and exit, plus say a VPIRRegionBlock representing the entire original scalar loop: https://llvm.org/devmtg/2023-10/slides/techtalks/Hahn-VPlan-StatusUpdateAndRoadmap.pdf#page=37 

Adding some comments inline.

https://github.com/llvm/llvm-project/pull/93398