[VPlan] Update VPValue::getDef to return VPRecipeBase, adjust name(NFC)

The return value of getDef is guaranteed to be a VPRecipeBase and all
users can also accept a VPRecipeBase *. Most users actually case to
VPRecipeBase or a specific recipe before using it, so this change
removes a number of redundant casts.

Also rename it to getDefiningRecipe to make the name a bit clearer.

Reviewed By: Ayal

Differential Revision: https://reviews.llvm.org/D136068