Functions: add dead node removal and constant folding optimization

Those optimizations work on the multi-function network level.
Not only will they make the network evaluation faster, but they also
simplify the network a lot. That makes it easier to understand the
exported dot graph.

source/blender/functions/FN_multi_function_network_optimization.hh

@@ -0,0 +1,31 @@
+/*
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version 2
+ * of the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+ */
+
+#ifndef __FN_MULTI_FUNCTION_NETWORK_OPTIMIZATION_HH__
+#define __FN_MULTI_FUNCTION_NETWORK_OPTIMIZATION_HH__
+
+#include "FN_multi_function_network.hh"
+
+#include "BLI_resource_collector.hh"
+
+namespace blender::fn::mf_network_optimization {
+
+void dead_node_removal(MFNetwork &network);
+void constant_folding(MFNetwork &network, ResourceCollector &resources);
+
+}  // namespace blender::fn::mf_network_optimization
+
+#endif /* __FN_MULTI_FUNCTION_NETWORK_OPTIMIZATION_HH__ */