Auto merge of #37979 - nnethercote:Ty-super_fold_with, r=eddyb

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)
```

src/librustc/ty/structural_impls.rs

@@ -481,7 +481,12 @@ impl<'tcx> TypeFoldable<'tcx> for Ty<'tcx> {
             ty::TyUint(_) | ty::TyFloat(_) | ty::TyError | ty::TyInfer(_) |
             ty::TyParam(..) | ty::TyNever => return self
         };
-        folder.tcx().mk_ty(sty)
+
+        if self.sty == sty {
+            self
+        } else {
+            folder.tcx().mk_ty(sty)
+        }
     }
 
     fn fold_with<'gcx: 'tcx, F: TypeFolder<'gcx, 'tcx>>(&self, folder: &mut F) -> Self {