$clog2() should return an integer value.

The constant $clog2() was returning a value that had an integer
width, but it did not have the sign bit set. This could result
in incorrect math expressions. This patch sets the sign bit for
the constant $clog2() result.

eval_tree.cc

@@ -1703,6 +1703,7 @@ NetExpr* evaluate_clog2(NetExpr*&arg_)
 		  res = integer_width;
 
 	    verinum tmp (res, integer_width);
+	    tmp.has_sign(true);
 	    NetEConst*rtn = new NetEConst(tmp);
 	    return rtn;
       }