[VPlan] Replace VPRegionBlock with explicit CFG before execute (NFCI). (#117506)

Building on top of https://github.com/llvm/llvm-project/pull/114305,
replace VPRegionBlocks with explicit CFG before executing.

This brings the final VPlan closer to the IR that is generated and
helps to simplify codegen.

It will also enable further simplifications of phi handling during
execution and transformations that do not have to preserve the 
canonical IV required by loop regions. This for example could include
replacing the canonical IV with an EVL based phi while completely
removing the original canonical IV.

PR: https://github.com/llvm/llvm-project/pull/117506