[VPlan] Hook IR blocks into VPlan during skeleton creation (NFC) (#114292)
As a first step to move towards modeling the full skeleton in VPlan,
start by wrapping IR blocks created during legacy skeleton creation in
VPIRBasicBlocks and hook them into the VPlan. This means the skeleton
CFG is represented in VPlan, just before execute. This allows moving
parts of skeleton creation into recipes in the VPBBs gradually.
Note that this allows retiring some manual DT updates, as this will be
handled automatically during VPlan execution.
PR: https://github.com/llvm/llvm-project/pull/114292