Codechange: Speed up industry generation using industry-type checks. (#13094)

Store a list of industries per industry type. This allows industry generation checks which only consider a specific industry type to check a reduced set of industries, leading to a potential performance increase.

This also removes the need to track industry type counts as well.

src/saveload/oldloader_sl.cpp

@@ -874,7 +874,7 @@ static bool LoadOldIndustry(LoadgameState *ls, int num)
 			i->random_colour = RemapTTOColour(i->random_colour);
 		}
 
-		Industry::IncIndustryTypeCount(i->type);
+		Industry::industries[i->type].push_back(i->index); // Assume savegame indices are sorted.
 	} else {
 		delete i;
 	}