Add retime test

tests/simple/retime.v

@@ -0,0 +1,6 @@
+module retime_test(input clk, input [7:0] a, output z);
+    reg [7:0] ff = 8'hF5;
+    always @(posedge clk)
+        ff <= {ff[6:0], ^a};
+    assign z = ff[7];
+endmodule