Recommit "[LV] Allow tryToCreateWidenRecipe to return a VPValue, use for blends."

This reverts the revert commit 437f0bbc.

It adds a new toVPRecipeResult, which forces VPRecipeOrVPValueTy to be
constructed with a VPRecipeBase *. This should address ambiguous
constructor issues for recipe sub-types that also inherit from VPValue.