[VPlan] Treat VPVector(End)PointerRecipe as single-scalar, if ops are. (#169249)
VPVector(End)PointerRecipes are single-scalar if all their operands are.
This should be effectively NFC currently, but it should re-enable cost
checking for some more VPWidenMemoryRecipe after
https://github.com/llvm/llvm-project/pull/157387 as discovered by
John Brawn.