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.

module.cpp

@@ -273,7 +273,7 @@ Module::AddGlobalVariable(Symbol *sym, Expr *initExpr, bool isConst) {
                 initExpr = TypeConvertExpr(initExpr, sym->type, "initializer");
             
             if (initExpr != NULL) {
-                initExpr = initExpr->Optimize();
+                initExpr = Optimize(initExpr);
                 // Fingers crossed, now let's see if we've got a
                 // constant value..
                 llvmInitializer = initExpr->GetConstant(sym->type);