Cast values to a dtype that algos.pad and algos.backfill can handle.

def _cast_values_for_fillna(values, dtype):
    """
    Cast values to a dtype that algos.pad and algos.backfill can handle.
    """
    # TODO: for int-dtypes we make a copy, but for everything else this
    #  alters the values in-place.  Is this intentional?

    if (is_datetime64_dtype(dtype) or is_datetime64tz_dtype(dtype) or
            is_timedelta64_dtype(dtype)):
        values = values.view(np.int64)

    elif is_integer_dtype(values):
        # NB: this check needs to come after the datetime64 check above
        values = ensure_float64(values)

    return values