In the Verilog backend, only sort modules that we're going to emit.

If you have a large design with a lot of modules and you use the Verilog
backend to emit modules one at a time to separate files, performance is
very low. The problem is that the Verilog backend calls `design->sort()`
every time, which sorts the contents of all modules, and this is slow
even when everything is already sorted.

We can easily fix this by only sorting the contents of modules that
we're actually going to emit.

backends/verilog/verilog_backend.cc

@@ -2612,7 +2612,7 @@ struct VerilogBackend : public Backend {
 		Pass::call(design, "clean_zerowidth");
 		log_pop();
 
-		design->sort();
+               design->sort_modules();
 
 		*f << stringf("/* Generated by %s */\n", yosys_maybe_version());
 
@@ -2625,6 +2625,7 @@ struct VerilogBackend : public Backend {
 				continue;
 			}
 			log("Dumping module `%s'.\n", module->name.c_str());
+			module->sort();
 			dump_module(*f, "", module);
 		}