Do not replace explicit parameters with implicit wires.

During elaborate_sig, the code to detect implicit wires did not
properly account for the possibility that the name is that of an
explicitly declared parameter.

elab_sig.cc

@@ -261,6 +261,9 @@ bool PEIdent::elaborate_sig(Design*des, NetScope*scope) const
       if (eve != 0)
 	    return false;
 
+      if (par != 0)
+	    return true;
+
       if (sig == 0)
 	    sig = make_implicit_net_(des, scope);