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: #3034
Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>

lib/path.js

@@ -408,7 +408,9 @@ var posix = {};
 
 
 function posixSplitPath(filename) {
-  return splitPathRe.exec(filename).slice(1);
+  const out = splitPathRe.exec(filename);
+  out.shift();
+  return out;
 }