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).

stmt.cpp

@@ -997,12 +997,12 @@ lVaryingBCPreFunc(ASTNode *node, void *d) {
 /** Postorder callback function for checking for varying breaks or
     continues; decrement the varying control flow depth after the node's
     children have been processed, if this is a varying if statement. */
-static bool
+static ASTNode *
 lVaryingBCPostFunc(ASTNode *node, void *d) {
     VaryingBCCheckInfo *info = (VaryingBCCheckInfo *)d;
     if (lIsVaryingFor(node))
         --info->varyingControlFlowDepth;
-    return true;
+    return node;
 }