Invalidate fastpath when calling attr_reader by super

The same bug as 8355a99883 existed in attr_reader too.

test/ruby/test_super.rb

@@ -604,7 +604,7 @@ class TestSuper < Test::Unit::TestCase
     assert_equal :boo2, subklass.new.boo
   end
 
-  def test_super_attr_writer # Bug #16785
+  def test_super_attr_writer # [Bug #16785]
     writer_class = Class.new do
       attr_writer :test
     end
@@ -633,4 +633,31 @@ class TestSuper < Test::Unit::TestCase
     assert_equal 3, superwriter.instance_variable_get(:@test)
     assert_equal 4, inherited.instance_variable_get(:@test)
   end
+
+  def test_super_attr_reader
+    writer_class = Class.new do
+      attr_reader :test
+    end
+    superwriter_class = Class.new(writer_class) do
+      def initialize
+        @test = 1 # index: 1
+      end
+
+      def test
+        super
+      end
+    end
+    inherited_class = Class.new(superwriter_class) do
+      def initialize
+        @a = nil
+        @test = 2 # index: 2
+      end
+    end
+
+    superwriter = superwriter_class.new
+    assert_equal 1, superwriter.test # set ic->index of superwriter_class#test to 1
+
+    inherited = inherited_class.new
+    assert_equal 2, inherited.test # it may read index=1 while it should be index=2
+  end
 end

vm_insnhelper.c

@@ -3235,9 +3235,9 @@ vm_search_super_method(const rb_control_frame_t *reg_cfp, struct rb_call_data *c
             switch (cached_cme->def->type) {
               // vm_call_refined (search_refined_method) assumes cc->call is vm_call_super_method on invokesuper
               case VM_METHOD_TYPE_REFINED:
-              // cc->klass is superclass of a class of receiver. Checking cc->klass is not enough to invalidate IVC for the receiver class.
+              // cc->klass is superclass of receiver class. Checking cc->klass is not enough to invalidate IVC for the receiver class.
               case VM_METHOD_TYPE_ATTRSET:
-              // TODO: case VM_METHOD_TYPE_IVAR:
+              case VM_METHOD_TYPE_IVAR:
                 vm_cc_call_set(cd->cc, vm_call_super_method); // invalidate fastpath
                 break;
               default: