Auto merge of #108020 - nnethercote:opt-mk_region, r=compiler-errors

Optimize `mk_region`

PR #107869 avoiding some interning under `mk_ty` by special-casing `Ty` variants with simple (integer) bodies. This PR does something similar for regions.

r? `@compiler-errors`

compiler/rustc_infer/src/infer/mod.rs

@@ -1093,7 +1093,7 @@ impl<'tcx> InferCtxt<'tcx> {
     ) -> ty::Region<'tcx> {
         let region_var =
             self.inner.borrow_mut().unwrap_region_constraints().new_region_var(universe, origin);
-        self.tcx.mk_region(ty::ReVar(region_var))
+        self.tcx.mk_re_var(region_var)
     }
 
     /// Return the universe that the region `r` was created in. For