url: fix performance regression
This (mostly) fixes a performance regression introduced in e1e669b1. While this PR does help, it doesn't completely restore performance levels pre- e1e669b1 but it's fairly close.
Example benchmark results:
confidence improvement accuracy (*) (**) (***)
url/legacy-vs-whatwg-url-get-prop.js e=11 method='whatwg' type='auth' *** 11.87 % ±1.67% ±2.24% ±2.94%
/cc @jasnell