[VPlan] Implement cloning of VPlans. (#73158)

This patch implements cloning for VPlans and recipes. Cloning is used in
the epilogue vectorization path, to clone the VPlan for the main vector
loop. This means we won't re-use a VPlan when executing the VPlan for
the epilogue vector loop, which in turn will enable us to perform
optimizations based on UF & VF.