Auto merge of #145851 - lolbinarycat:rustdoc-optimize, r=GuillaumeGomez

rustdoc: a few micro-optimizations targeted at build_impl

Unsure if these will be anything substantial, but the first one at least should git rid of quite a few branches, second one unsure if it's worth it.

r? `@GuillaumeGomez`

Author: bors