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

[via llvm-commits]

ayalz wrote:

Note that Case A of VPBasicBlock::execute() which reuses existing IR BB's is relieved by this patch from dealing with the pre-preheader, but is still needed for handling the preheader. When the preheader is connected to the pre-preheader, through bypass blocks, Case A can retire.

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