Fix order of normalization and recursion in const folding.

Fixes rust-lang#126831.

Without this patch, type normalization is not always idempotent, which
leads to all sorts of bugs in places that assume that normalizing a
normalized type does nothing.

compiler/rustc_trait_selection/src/traits/query/normalize.rs

@@ -333,14 +333,14 @@ impl<'cx, 'tcx> FallibleTypeFolder<TyCtxt<'tcx>> for QueryNormalizer<'cx, 'tcx>
             return Ok(constant);
         }
 
-        let constant = constant.try_super_fold_with(self)?;
-        debug!(?constant, ?self.param_env);
-        Ok(crate::traits::with_replaced_escaping_bound_vars(
+        let constant = crate::traits::with_replaced_escaping_bound_vars(
             self.infcx,
             &mut self.universes,
             constant,
             |constant| constant.normalize(self.infcx.tcx, self.param_env),
-        ))
+        );
+        debug!(?constant, ?self.param_env);
+        constant.try_super_fold_with(self)
     }
 
     #[inline]