Cleanup how we handle side effects in HInstruction and put the right flags for HIs, modulo, truncat…

tests/compiler/dart2js/gvn_test.dart

 // Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 import 'compiler_helper.dart';
 
 const String TEST_ONE = r"""
 void foo(bar) {
   for (int i = 0; i < 1; i++) {
     print(1 + bar);
     print(1 + bar);
   }
 }
 """;
 
+// Check that modulo does not have any side effect and we are
+// GVN'ing the length of [:list:].
+const String TEST_TWO = r"""
+void foo(a) {
+  var list = new List<int>();
+  list[0] = list[0 % a];
+  list[1] = list[1 % a];
+}
+""";
+
 main() {
   String generated = compile(TEST_ONE, entry: 'foo');
   RegExp regexp = new RegExp(r"1 \+ [a-z]+");
-  Iterator matches = regexp.allMatches(generated).iterator;
-  Expect.isTrue(matches.moveNext());
-  Expect.isFalse(matches.moveNext());
+  checkNumberOfMatches(regexp.allMatches(generated).iterator, 1);
+
+  generated = compile(TEST_TWO, entry: 'foo');
+  checkNumberOfMatches(new RegExp("length").allMatches(generated).iterator, 1);
 }