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

[Florian Hahn via llvm-commits]

https://github.com/fhahn commented:

> Looks right to me, thanks!
> 
> A subclass looks better than adding a bit to VPBasicBlock, right?
> 

Yes I think so, that way we can keep things separate (although most logic is shared)

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

Updated, thanks!



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