Move function codegen data from IrFunction to new FuncGen.

This change reduces memory consumption significantly by releasing the
memory held by the STL containers that are now inside FuncGen.

gen/functions.cpp

@@ -774,7 +774,10 @@ void DtoDefineFunction(FuncDeclaration* fd)
     }
 
     // output function body
+    irfunction->gen = new FuncGen;
     fd->fbody->toIR(gIR);
+    delete irfunction->gen;
+    irfunction->gen = 0;
 
     // TODO: clean up this mess