[Turbofan] Too-strong CHECK during optimization.

test/mjsunit/regress/regress-crbug-630952.js

+// Copyright 2016 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: --always-opt
+try {
+function __f_4(sign_bit,
+                          mantissa_29_bits) {
+}
+__f_4.prototype.returnSpecial = function() {
+                     this.mantissa_29_bits * mantissa_29_shift;
+}
+__f_4.prototype.toSingle = function() {
+  if (-65535) return this.toSingleSubnormal();
+}
+__f_4.prototype.toSingleSubnormal = function() {
+  if (__v_15) {
+      var __v_7 = this.mantissa_29_bits == -1 &&
+                 (__v_13 & __v_10 ) == 0;
+    }
+  __v_8 >>= __v_7;
+}
+__v_14 = new __f_4();
+__v_14.toSingle();
+} catch(e) {}