[turbofan] Split pure nodes in the scheduler if beneficial.

test/mjsunit/asm/redundancy1.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: --turbo-splitting
+
+function module(stdlib, foreign, heap) {
+    "use asm";
+    function foo(i) {
+      var j = 0;
+      i = i|0;
+      if (i < 0) {
+        j = i+1|0;
+      }
+      if (i > 0) {
+        j = i+1|0;
+      }
+      return j;
+    }
+    return { foo: foo };
+}
+
+var foo = module(this, {}, new ArrayBuffer(64*1024)).foo;
+assertEquals(0, foo(0));
+assertEquals(0, foo(-1));
+assertEquals(12, foo(11));