Fix LoadFieldByIndex to take mutable heap-numbers into account.

test/mjsunit/fuzz-natives-part1.js

 // Copyright 2011 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 164 matching lines @@
 
   // Only applicable to strings.
   "_HasCachedArrayIndex": true,
   "_GetCachedArrayIndex": true,
   "_OneByteSeqStringSetChar": true,
   "_TwoByteSeqStringSetChar": true,
 
   // Only applicable to TypedArrays.
   "_TypedArrayInitialize": true,
 
+  // Only applicable to loading mutable doubles.
+  "LoadMutableDouble": true,
+
   // Only applicable to generators.
   "_GeneratorNext": true,
   "_GeneratorThrow": true,
 
   // Only applicable to DataViews.
   "_DataViewInitialize": true,
 };
 
 var currentlyUncallable = {
   // We need to find a way to test this without breaking the system.
@@ skipping 12 matching lines @@
     if (name in knownProblems || name in currentlyUncallable)
       continue;
     print(name);
     var argc = nativeInfo[1];
     testArgumentCount(name, argc);
     testArgumentTypes(name, argc);
   }
 }
 
 testNatives();