Avoid more unnecessary mk_ty calls in Ty::super_fold_with.
This speeds up several rustc-benchmarks by 1--5%.
This PR is the lovechild of #37108 and #37705.
```
futures-rs-test 4.059s vs 4.011s --> 1.012x faster (variance: 1.016x, 1.026x)
helloworld 0.236s vs 0.239s --> 0.986x faster (variance: 1.051x, 1.014x)
html5ever-2016- 3.831s vs 3.824s --> 1.002x faster (variance: 1.020x, 1.019x)
hyper.0.5.0 4.928s vs 4.936s --> 0.998x faster (variance: 1.003x, 1.012x)
inflate-0.1.0 4.135s vs 4.104s --> 1.007x faster (variance: 1.026x, 1.028x)
issue-32062-equ 0.309s vs 0.303s --> 1.017x faster (variance: 1.019x, 1.084x)
issue-32278-big 1.818s vs 1.797s --> 1.011x faster (variance: 1.011x, 1.008x)
jld-day15-parse 1.304s vs 1.271s --> 1.026x faster (variance: 1.018x, 1.012x)
piston-image-0. 10.938s vs 10.921s --> 1.002x faster (variance: 1.025x, 1.016x)
reddit-stress 2.327s vs 2.208s --> 1.054x faster (variance: 1.016x, 1.006x)
regex-0.1.80 8.796s vs 8.727s --> 1.008x faster (variance: 1.012x, 1.019x)
regex.0.1.30 2.294s vs 2.249s --> 1.020x faster (variance: 1.013x, 1.026x)
rust-encoding-0 1.914s vs 1.886s --> 1.015x faster (variance: 1.027x, 1.026x)
```