Perf regression in vbench replace_fillna() in v0.10.1 vs v0.10.0
![perf](https://f.cloud.github.com/assets/1820866/377686/b9238e6e-a4de-11e2-8106-dd2d8c4e3040.png)
![frame_fillna_many_col_pad](https://f.cloud.github.com/assets/1820866/377701/4232ef00-a4e0-11e2-883f-d4beacadd0b0.png)
![reindex_fillna_backfill](https://f.cloud.github.com/assets/1820866/377767/39945492-a4ea-11e2-9fc8-0d16debbc5e5.png)

```
commit     timing[ms]
f7a11d6    14.730644
3700b70    14.744759
87126b9    14.725828
f971840    16.744852
0aa5eea    16.848993
```
