Only set binary operation side effects flags, when observable.

test/mjsunit/regress/regress-binop.js

 // Copyright 2013 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 148 matching lines @@
 assertEquals(t2(2,null), 2/0);
 assertEquals(t2(-2,null), -2/0);
 assertEquals(t2(1,2.4), 1/2.4);
 assertEquals(t2(1.3,2.4), 1.3/2.4);
 assertEquals(t2(null,2.4), 0/2.4);
 assertEquals(t2(1.3,null), 1.3/0);
 assertEquals(t2(undefined,2), NaN/2);
 assertEquals(t2(1,1<<30), 1/(1<<30));
 assertEquals(t2(1,2), 1/2);
 
+
+// Assert that the hole is not truncated to nan for string add.
+function string_add(a,i) {
+  var d = [0.1, ,0.3];
+  return a + d[i];
+}
+
+string_add(1.1, 0);
+string_add("", 0);
+%OptimizeFunctionOnNextCall(string_add);
+string_add(1.1, 0);
+// There comes the hole
+assertEquals("undefined", string_add("", 1));