[VPlan] Add createWiden{Load,Store} VPBuilder members (NFC) (#210543)
Add member functions to create wide load/store and migrate various sites
to use them.
It also updates makeMemOpWideningDecisions to use it. For consistency,
ReplaceWith has been updated to always require inserted recipes.
PR: https://github.com/llvm/llvm-project/pull/210543