Fixed handling of power operator

tests/simple/constpower.v

@@ -0,0 +1,15 @@
+module constpower(ys, yu);
+
+output [8*8*8-1:0] ys, yu;
+
+genvar i, j;
+
+generate
+	for (i = 0; i < 8; i = i+1)
+	for (j = 0; j < 8; j = j+1) begin:V
+		assign ys[i*8 + j*64 + 7 : i*8 + j*64] = $signed(i-4) ** $signed(j-4);
+		assign yu[i*8 + j*64 + 7 : i*8 + j*64] = $unsigned(i) ** $unsigned(j);
+	end
+endgenerate
+
+endmodule