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

[Florian Hahn via llvm-commits]

https://github.com/fhahn commented:

> Last set of minor comments; time to add the test changes? Could the current tests be retained, isolating the changes that affect them to a separate patch?
> 

Updated the tests now. We could manually set the name to `ph`, but it's probably cleaner/explicit to include the adjusted name already in this patch

> Commit message should be updated to refer to VPIRBasicBlock, and fix typo: "startingt".
 Fixed, thanks!

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