Add #1598 testcase

tests/arch/xilinx/bug1598.ys

@@ -0,0 +1,16 @@
+read_verilog <<EOT
+module led_blink (
+        input clk,
+        output ledc
+    );
+ 
+    reg [6:0] led_counter = 0;
+    always @( posedge clk ) begin
+            led_counter <= led_counter + 1;
+    end
+    assign ledc = !led_counter[ 6:3 ];
+ 
+endmodule
+EOT
+proc
+equiv_opt -assert -map +/xilinx/cells_sim.v synth_xilinx -abc9