inliner: Use substs_for_mir_body

Changes from 68965 extended the kind of instances that are being
inlined. For some of those, the `instance_mir` returns a MIR body that
is already expressed in terms of the types found in substitution array,
and doesn't need further substitution.

Use `substs_for_mir_body` to take that into account.

compiler/rustc_codegen_ssa/src/mir/mod.rs

@@ -92,15 +92,11 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
         T: Copy + TypeFoldable<'tcx>,
     {
         debug!("monomorphize: self.instance={:?}", self.instance);
-        if let Some(substs) = self.instance.substs_for_mir_body() {
-            self.cx.tcx().subst_and_normalize_erasing_regions(
-                substs,
-                ty::ParamEnv::reveal_all(),
-                &value,
-            )
-        } else {
-            self.cx.tcx().normalize_erasing_regions(ty::ParamEnv::reveal_all(), *value)
-        }
+        self.instance.subst_mir_and_normalize_erasing_regions(
+            self.cx.tcx(),
+            ty::ParamEnv::reveal_all(),
+            value,
+        )
     }
 }