[turbofan] Try hard(er) to use smi representation for float64 values.

test/mjsunit/regress/regress-crbug-485410.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
 
 var doubles = new Float64Array(1);
 function ToHeapNumber(i) {
   doubles[0] = i;
   return doubles[0];
 }
 for (var i = 0; i < 3; i++) ToHeapNumber(i);
 %OptimizeFunctionOnNextCall(ToHeapNumber);
-assertFalse(%IsSmi(ToHeapNumber(1)));
+ToHeapNumber(1);
 
 function Fail(a, i, v) {
   a[i] = v;
 }
 
 for (var i = 0; i < 3; i++) Fail(new Array(1), 1, i);
 %OptimizeFunctionOnNextCall(Fail);
 // 1050 > JSObject::kMaxGap, causing stub failure and runtime call.
 Fail(new Array(1), ToHeapNumber(1050), 3);