Have WalkAST postorder callback function return an ASTNode *

In general, it should just return the original node pointer, but for type checking
and optimization passes, it can return a new value for the node (that will be
assigned where the old one was in the tree.)

Along the way, fixed some bugs in WalkAST() where the postorder callback wouldn't
end up being called for a few expr types (sizeof, dereference, address of, 
reference).

func.cpp

@@ -387,9 +387,8 @@ Function::GenerateIR() {
     SourcePos firstStmtPos = sym->pos;
     if (code) {
         StmtList *sl = dynamic_cast<StmtList *>(code);
-        if (sl && sl->GetStatements().size() > 0 && 
-            sl->GetStatements()[0] != NULL)
-            firstStmtPos = sl->GetStatements()[0]->pos;
+        if (sl && sl->stmts.size() > 0 && sl->stmts[0] != NULL)
+            firstStmtPos = sl->stmts[0]->pos;
         else
             firstStmtPos = code->pos;
     }