implement binop-assignment (+= etc.)

test/xorshift.cwa

@@ -5,4 +5,11 @@ fn xorshift(x: i32) -> i32 {
     let lazy x = x ^ (x #>> 17);
     let inline x = x ^ (x << 5);
     x
+}
+
+fn xorshift2(x: i32) -> i32 {
+    x ^= x << 13;
+    x ^= x #>> 17;
+    x ^= x << 5;
+    x
 }