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.h

@@ -53,10 +53,11 @@ public:
     virtual ~ASTNode();
 
     /** The Optimize() method should perform any appropriate early-stage
-        optimizations on the node (e.g. constant folding).  The caller
-        should use the returned ASTNode * in place of the original node.
-        This method may return NULL if an error is encountered during
-        optimization. */
+        optimizations on the node (e.g. constant folding).  This method
+        will be called after the node's children have already been
+        optimized, and the caller will store the returned ASTNode * in
+        place of the original node.  This method should return NULL if an
+        error is encountered during optimization. */
     virtual ASTNode *Optimize() = 0;
 
     /** Type checking should be performed by the node when this method is
@@ -112,4 +113,8 @@ typedef ASTNode * (* ASTPostCallBackFunc)(ASTNode *node, void *data);
 extern ASTNode *WalkAST(ASTNode *root, ASTPreCallBackFunc preFunc,
                         ASTPostCallBackFunc postFunc, void *data);
 
+extern Expr *Optimize(Expr *);
+extern Stmt *Optimize(Stmt *);
+extern ASTNode *Optimize(ASTNode *root);
+
 #endif // ISPC_AST_H