Normalize MIR locals' types for generator layout computation.

src/test/ui/repeat_count_const_in_async_fn.rs

@@ -0,0 +1,10 @@
+// check-pass
+// edition:2018
+// compile-flags: --crate-type=lib
+
+pub async fn test() {
+    const C: usize = 4;
+    foo(&mut [0u8; C]).await;
+}
+
+async fn foo(_: &mut [u8]) {}