Rewrite AST optimization infrastructure to be built on top of WalkAST().

Specifically, stmts and exprs are no longer responsible for first recursively
optimizing their children before doing their own optimization (this turned
out to be error-prone, with children sometimes being forgotten.)  They now
are just responsible for their own optimization, when appropriate.

ast.cpp

@@ -225,3 +225,27 @@ WalkAST(ASTNode *node, ASTPreCallBackFunc preFunc, ASTPostCallBackFunc postFunc,
     else
         return node;
 }
+
+
+static ASTNode *
+lOptimizeNode(ASTNode *node, void *) {
+    return node->Optimize();
+}
+
+
+ASTNode *
+Optimize(ASTNode *root) {
+    return WalkAST(root, NULL, lOptimizeNode, NULL);
+}
+
+
+Expr *
+Optimize(Expr *expr) {
+    return (Expr *)Optimize((ASTNode *)expr);
+}
+
+
+Stmt *
+Optimize(Stmt *stmt) {
+    return (Stmt *)Optimize((ASTNode *)stmt);
+}