add regression test for aliasing in product rewritting

test/product.h

diff --git a/test/product.h b/test/product.h
index 3b65112..0425a92 100644
--- a/test/product.h
+++ b/test/product.h
@@ -216,6 +216,8 @@
     // CwiseBinaryOp
     VERIFY_IS_APPROX(x = y + A*x, A*z);
     x = z;
+    VERIFY_IS_APPROX(x = y - A*x, A*(-z));
+    x = z;
     // CwiseUnaryOp
     VERIFY_IS_APPROX(x = Scalar(1.)*(A*x), A*z);
   }