Optimize `TyCtxt::hygienic_eq`.
Optimize `TyCtxt::hygienic_eq`.

It's measurably faster to avoid the context comparison when possible.

The commit also adds `hygienic_eq` in one appropriate place.

aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust