[VPlan] Simplify branch on False in VPlan transform (NFC). (#140409)
Simplify branch on false, starting with the branch from the middle block
to the scalar preheader. Initially this helps simplifying the initial
VPlan construction.
Depends on https://github.com/llvm/llvm-project/pull/140405.
PR: https://github.com/llvm/llvm-project/pull/140409