Do fix_*_builtin_expr hacks on the writeback results

tests/ui/traits/new-solver/normalized-const-built-in-op.rs

@@ -0,0 +1,11 @@
+// compile-flags: -Ztrait-solver=next
+// check-pass
+
+const fn foo() {
+    let mut x = [1, 2, 3];
+    // We need to fix up `<<[i32; 3] as Index<usize>>::Output as AddAssign>`
+    // to be treated like a built-in operation.
+    x[1] += 5;
+}
+
+fn main() {}