Avoid unnecessary mk_ty calls in Ty::super_fold_with.
Avoid unnecessary mk_ty calls in Ty::super_fold_with.

This speeds up compilation of several rustc-benchmarks by 1--2% and the
workload in #36799 by 5%.

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