Don't include foreach stmts in cost estimates from EstimateCost().

Because they reestablish an 'all on' mask inside their body, it doesn't
make sense to include their cost when evaluating whether it's worth
re-establishing an 'all on' mask dynamically.  (This does mean that
EstimateCost()'s return value isn't the most obvious thing, but currently
in all the cases where we need it, this is the more appropriate value to
return.)

ast.cpp

@@ -307,19 +307,39 @@ TypeCheck(Stmt *stmt) {
 }
 
 
+struct CostData {
+    CostData() { cost = foreachDepth = 0; }
+
+    int cost;
+    int foreachDepth;
+};
+
+
 static bool
-lCostCallback(ASTNode *node, void *c) {
-    int *cost = (int *)c;
-    *cost += node->EstimateCost();
+lCostCallbackPre(ASTNode *node, void *d) {
+    CostData *data = (CostData *)d;
+    if (dynamic_cast<ForeachStmt *>(node) != NULL)
+        ++data->foreachDepth;
+    if (data->foreachDepth == 0)
+        data->cost += node->EstimateCost();
     return true;
 }
 
 
+static ASTNode *
+lCostCallbackPost(ASTNode *node, void *d) {
+    CostData *data = (CostData *)d;
+    if (dynamic_cast<ForeachStmt *>(node) != NULL)
+        --data->foreachDepth;
+    return node;
+}
+
+
 int
 EstimateCost(ASTNode *root) {
-    int cost = 0;
-    WalkAST(root, lCostCallback, NULL, &cost);
-    return cost;
+    CostData data;
+    WalkAST(root, lCostCallbackPre, lCostCallbackPost, &data);
+    return data.cost;
 }