[VPlan] Split VPWidenMemoryInstructionRecipe (NFCI). (#87411)

This patch introduces a new VPWidenMemoryRecipe base class and distinct
sub-classes to model loads and stores.

This is a first step in an effort to simplify and modularize code
generation for widened loads and stores and enable adding further more
specialized memory recipes.

PR: https://github.com/llvm/llvm-project/pull/87411