cleanup some skip_binder -> subst_identity

compiler/rustc_middle/src/ty/instance.rs

@@ -586,7 +586,7 @@ impl<'tcx> Instance<'tcx> {
         if let Some(substs) = self.substs_for_mir_body() {
             v.subst(tcx, substs)
         } else {
-            v.skip_binder()
+            v.subst_identity()
         }
     }
 

compiler/rustc_mir_transform/src/shim.rs

@@ -647,7 +647,7 @@ fn build_call_shim<'tcx>(
     let mut sig = if let Some(sig_substs) = sig_substs {
         sig.subst(tcx, &sig_substs)
     } else {
-        sig.skip_binder()
+        sig.subst_identity()
     };
 
     if let CallKind::Indirect(fnty) = call_kind {