Add Selection::clear() method

Use method in `select.cc` to reduce code duplication.

kernel/rtlil.cc

@@ -891,6 +891,14 @@ void RTLIL::Selection::optimize(RTLIL::Design *design)
 	}
 }
 
+void RTLIL::Selection::clear()
+{
+	full_selection = false;
+	complete_selection = false;
+	selected_modules.clear();
+	selected_members.clear();
+}
+
 RTLIL::Design::Design()
   : verilog_defines (new define_map_t)
 {

kernel/rtlil.h

@@ -1241,6 +1241,9 @@ struct RTLIL::Selection
 		return !selects_all() && selected_modules.empty() && selected_members.empty();
 	}
 
+	// clear this selection, leaving it empty
+	void clear();
+
 	// create a new selection which is empty
 	static Selection EmptySelection(RTLIL::Design *design = nullptr) { return Selection(false, false, design); };