TyCtxtEnsure::const_eval_poly: Don't try to erase regions twice

Probably won't make a difference because erase_regions no-ops on region-erased things anyway.

compiler/rustc_middle/src/mir/interpret/queries.rs

@@ -209,7 +209,7 @@ impl<'tcx> TyCtxtEnsureOk<'tcx> {
         // into `const_eval` which will return `ErrorHandled::TooGeneric` if any of them are
         // encountered.
         let args = GenericArgs::identity_for_item(self.tcx, def_id);
-        let instance = ty::Instance::new(def_id, self.tcx.erase_regions(args));
+        let instance = ty::Instance::new(def_id, args);
         let cid = GlobalId { instance, promoted: None };
         let typing_env = ty::TypingEnv::post_analysis(self.tcx, def_id);
         // Const-eval shouldn't depend on lifetimes at all, so we can erase them, which should