[crankshaft] DCE must not eliminate (observable) math operations.

test/mjsunit/compiler/regress-4389-4.js

 // Copyright 2015 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// Flags: --allow-natives-syntax
+// Flags: --allow-natives-syntax --dead-code-elimination
 
-function foo(x) {
-  if (x === undefined) return;
-  while (true) {
-    while (1 || 2) { }
-    f();
-  }
-}
+function foo(x) { Math.round(x); }
+foo(1);
+foo(2);
 %OptimizeFunctionOnNextCall(foo);
-foo();
+assertThrows(function() { foo(Symbol()) }, TypeError);