path: improve posixSplitPath performance

Instead of slicing the first element off of the matches, shift and then
return. This improves performance of the following path functions:

- basename: 18-20%
- extname: 60-70%
- dirname: 18-20%
- parse: 20-25%

PR-URL: https://github.com/nodejs/node/pull/3034


Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>