[VPlan] Make simplifyRecipe more like InstCombine
Most combines in simplifyRecipe RAUW a value, but not all of them erase the old recipe.
Unify them and bring it in line with InstCombine by having it return a VPValue, which simplifyRecipes can then call RAUW with, and automatically erase the old recipe.
Similarly to InstCombine, combines that modify a recipe should return the same recipe.
