Fix beyond-heap load on x64 Crankshafted StringCharFromCode

src/x64/lithium-codegen-x64.cc

 // Copyright 2013 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 4366 matching lines @@
   DeferredStringCharFromCode* deferred =
       new(zone()) DeferredStringCharFromCode(this, instr);
 
   ASSERT(instr->hydrogen()->value()->representation().IsInteger32());
   Register char_code = ToRegister(instr->char_code());
   Register result = ToRegister(instr->result());
   ASSERT(!char_code.is(result));
 
   __ cmpl(char_code, Immediate(String::kMaxOneByteCharCode));
   __ j(above, deferred->entry());
+  __ movsxlq(char_code, char_code);
   __ LoadRoot(result, Heap::kSingleCharacterStringCacheRootIndex);
   __ movq(result, FieldOperand(result,
                                char_code, times_pointer_size,
                                FixedArray::kHeaderSize));
   __ CompareRoot(result, Heap::kUndefinedValueRootIndex);
   __ j(equal, deferred->entry());
   __ bind(deferred->exit());
 }
 
 
@@ skipping 1298 matching lines @@
                                FixedArray::kHeaderSize - kPointerSize));
   __ bind(&done);
 }
 
 
 #undef __
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_X64