Functions: implement common subnetwork elimination optimization

This was the last of the three network optimizations I developed in
the functions branch. Common subnetwork elimination and constant
folding together can get rid of most unnecessary nodes.

source/blender/functions/FN_multi_function_network_optimization.hh

@@ -25,6 +25,7 @@ namespace blender::fn::mf_network_optimization {
 
 void dead_node_removal(MFNetwork &network);
 void constant_folding(MFNetwork &network, ResourceCollector &resources);
+void common_subnetwork_elimination(MFNetwork &network);
 
 }  // namespace blender::fn::mf_network_optimization