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

This patch adds a new special type of VPBasicBlock that wraps an
existing IR basic block. Recipes of the block get added before the
terminator of the wrapped IR basic block. Making it a subclass of
VPBasicBlock avoids duplicating various APIs to manage recipes in a
block, as well as makes sure the traversals filtering VPBasicBlocks
automatically apply as well.

Initially VPIRBasicBlock are only used for the pre-preheader (wrapping
the original preheader of the scalar loop).

As follow-up, this will be used to move more parts of the skeleton
inside VPlan, starting with the branch and condition in the middle
block.

Separated out of https://github.com/llvm/llvm-project/pull/92651

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