benchmark: convert var to es6 const

Converted var variable to es6 const to maintain
consistency with other benchmark files. Also clean up
the types array to make the files more succinct.

PR-URL: https://github.com/nodejs/node/pull/12886


Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>