Sparse get dummies perf
Previously, we did a scalar `elem == -1` for every element in the ndarray.

This replaces that check with a vectorized `array == -1`.

Running the ASV now. In the meantime, here's a simple timeit on the same problem

```python
# HEAD
In [3]: %timeit pd.get_dummies(s, sparse=True)
561 ms ± 4.96 ms per loop (mean ± std. dev. of 7 runs, 1 loop each)

# Master
In [3]: %timeit pd.get_dummies(s, sparse=True)
        2.18 s ± 273 ms per loop (mean ± std. dev. of 7 runs, 1 loop each)
```