[CINN] Fix eliminate if bug

Author: huangjiyi

paddle/cinn/operator_fusion/fusion_tracker/interpreter.cc

@@ -202,11 +202,10 @@ void RunReturnInstr(const std::shared_ptr<ReturnInstr>& instr,
     auto exprs = std::visit(FusibleOp2Expr(), fusion_op);
     for (auto expr : exprs) {
       std::string output_var_name = GetOutputTensor(expr)->name;
-      if (interpreter->global_var_names.count(output_var_name)) {
-        expr = ExprTransformerUtils::EliminateUselessIfTransformer()(expr);
-      } else {
+      if (!interpreter->global_var_names.count(output_var_name)) {
         expr = ExprTransformerUtils::RemoveAllAppendIfTransformer()(expr);
       }
+      expr = ExprTransformerUtils::EliminateUselessIfTransformer()(expr);
       result.push_back(expr);
     }
   }

test/ir/pir/cinn/test_horizontal_fusion.py

@@ -152,6 +152,20 @@ def init():
 
         self.check_accuracy_and_kernel_num(init, func, kernel_num=2)
 
+    def test_reduce_horizontal_fusion(self):
+        def func(x):
+            a = paddle.sum(x, axis=[0], keepdim=True)
+            a = paddle.reshape(a, [6])
+            b = paddle.sum(x, axis=[0], keepdim=True)
+            b = paddle.reshape(b, [6])
+            return a, b
+
+        def init():
+            x = paddle.rand((2, 2, 3))
+            return (x,)
+
+        self.check_accuracy_and_kernel_num(init, func, kernel_num=1)
+
 
 if __name__ == "__main__":
     unittest.main()