path: replace duplicate conditions by functions

It will also remove useless "code" variables by inlining
path.charCodeAt.

PR-URL: nodejs#18693
Reviewed-By: Ben Noordhuis <[email protected]>
Reviewed-By: Tobias Nießen <[email protected]>
Reviewed-By: Ruben Bridgewater <[email protected]>
Reviewed-By: Colin Ihrig <[email protected]>
Reviewed-By: Luigi Pinca <[email protected]>
Reviewed-By: Jon Moss <[email protected]>
Reviewed-By: Anatoli Papirovski <[email protected]>
Reviewed-By: James M Snell <[email protected]>