Core: Standardize indexOf comparisons

not present: `< 0`
present: `> -1`
at index: `=== N`

```
   raw     gz Compared to master @ 76df9e4e389d80bff410a9e5f08b848de1d21a2f    
    +7     +3 dist/jquery.js                                                   
    -1     -6 dist/jquery.min.js
```

(motivated primarily by consistency, secondarily by size [_various alternatives being larger, albeit by less than 10 bytes_], and tertiarily by a reluctance to also change Sizzle)