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).

ast.h

@@ -92,18 +92,24 @@ private:
 };
 
 
-/** Callback function type for the AST walk.
+/** Callback function type for preorder traversial visiting function for
+    the AST walk.
  */
-typedef bool (* ASTCallBackFunc)(ASTNode *node, void *data);
+typedef bool (* ASTPreCallBackFunc)(ASTNode *node, void *data);
+
+/** Callback function type for postorder traversial visiting function for
+    the AST walk.
+ */
+typedef ASTNode * (* ASTPostCallBackFunc)(ASTNode *node, void *data);
 
 /** Walk (some portion of) an AST, starting from the given root node.  At
     each node, if preFunc is non-NULL, call it, passing the given void
     *data pointer; if the call to preFunc function returns false, then the
-    children of the node aren't visited.  This then makes recursive calls
-    to WalkAST() to process the node's children; after doing so, calls
-    postFunc, at the node.  The return value from the postFunc call is
-    ignored. */
-extern void WalkAST(ASTNode *root, ASTCallBackFunc preFunc,
-                    ASTCallBackFunc postFunc, void *data);
+    children of the node aren't visited.  This function then makes
+    recursive calls to WalkAST() to process the node's children; after
+    doing so, calls postFunc, at the node.  The return value from the
+    postFunc call is ignored. */
+extern ASTNode *WalkAST(ASTNode *root, ASTPreCallBackFunc preFunc,
+                        ASTPostCallBackFunc postFunc, void *data);
 
 #endif // ISPC_AST_H