Rust compiler will realize that UnsafeCell::get can never actually point to null (references are never null) and the resulting assembly won't have null check.
That said, there is a problem with such an API as it's error-prone as NonNull is covariant. In most cases users are probably better using &UnsafeCell<T>. Is there any particular use-case for such a conversion method that you cannot do with &UnsafeCell<T>?
In terms of name of a method, I think get_non_null_ptr should be fine for an unstable method name. This can be changed while the method is unstable anyway.