Rollup merge of #147237 - yotamofek:pr/rustdoc/highlight/optimize-end_expansion, r=GuillaumeGomez

[rustdoc] Cleanup "highlight::end_expansion"

~Looks like a ~5% improvement on the highlight benchmark.
Obviously, highlighting is only a small part of rustdoc's runtime, so improvement won't be as large on rustc-perf (if there's even an improvement), but holding fingers for a nice gain.~

Perf seems neutral, but IMHO this is a nice small cleanup regardless.

r? `@GuillaumeGomez` (& perf run please!)

Author: Zalathar