X64: Adding macro to load double from memory, and use SSE3 instruction if present.

src/x64/macro-assembler-x64.h

 // 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
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   // Find the function context up the context chain.
   void LoadContext(Register dst, int context_chain_length);
 
   // Load the global function with the given index.
   void LoadGlobalFunction(int index, Register function);
 
   // Load the initial map from the global function. The registers
   // function and map can be the same.
   void LoadGlobalFunctionInitialMap(Register function, Register map);
 
+  // Loads a double value from memory to the low half of an XMMRegister.
+  // May or may not change the top half of the destionation register,
+  // depending on the opcode used.
+  void LoadDbl(XMMRegister dst, const Operand& src);
+
   // ---------------------------------------------------------------------------
   // Runtime calls
 
   // Call a code stub.
   void CallStub(CodeStub* stub);
 
   // Call a code stub and return the code object called.  Try to generate
   // the code if necessary.  Do not perform a GC but instead return a retry
   // after GC failure.
   MUST_USE_RESULT MaybeObject* TryCallStub(CodeStub* stub);
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       Jump(adaptor, RelocInfo::CODE_TARGET);
     }
     bind(&invoke);
   }
 }
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_X64_MACRO_ASSEMBLER_X64_H_