benchmark: refactor to eliminate redeclared vars

In order to comply with linting rules used in the rest of the code base,
eliminate redeclared variables. A conservative approach is used so as to
avoid unintentional performance issues (for example, as might be seen in
some situations when using `let` instead of `var`).

PR-URL: https://github.com/nodejs/node/pull/5468


Reviewed-By: Brian White <mscdex@mscdex.net>