Moving various Vector128/256 helper method to be implemented using other intrinsics...

Moving various Vector128/256 helper method to be implemented using other intrinsics (#21432)

* Updating Vector128.CreateScalar and Vector256.CreateScalar to be implemented using other intrinsics

* Updating Vector128.Equals and Vector256.Equals to be implemented using other intrinsics

* Updating Vector256.WithLower, Vector256.GetUpper, and Vector256.WithUpper to be implemented using other intrinsics

* Updating Vector128.Create(T) and Vector256.Create(T) to be implemented using other intrinsics

* Fixing the `NI_Base_Vector256_As` intrinsics to only fold the cast if AVX is supported and add a clarifying comment to the Vector128/256.Equals code

* Changing the various `*Software()` local functions in Vector128/256 to be `SoftwareFallback()`