[VPlan] Replace VPRecipeOrVPValue with VP2VP recipe simplification. (#76090)

Move simplification of VPBlendRecipes from early VPlan construction to
VPlan-to-VPlan based recipe simplification. This simplifies initial
construction.

Note that some in-loop reduction tests are failing at the moment, due to
the reduction predicate being created after the reduction recipe. I will
provide a patch for that soon.

PR: https://github.com/llvm/llvm-project/pull/76090