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

[via llvm-commits]

https://github.com/ayalz 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?

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

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