its not very significant for x86 (the only place this macro is currently enabled)

```
In [2]: a = np.ones(20000., dtype=np.bool)

In [3]: %timeit np.count_nonzero(a)
100000 loops, best of 3: 2.87 µs per loop

In [4]: %timeit np.count_nonzero(a[1:])
100000 loops, best of 3: 3.35 µs per loop

vs without this pr
In [4]: %timeit np.count_nonzero(a[1:])
100000 loops, best of 3: 19.4 µs per loop
```