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

src/x64/macro-assembler-x64.h

Index: src/x64/macro-assembler-x64.h
diff --git a/src/x64/macro-assembler-x64.h b/src/x64/macro-assembler-x64.h
index 4c177205b6be4f143933f751678641b9dbc14f52..7f8846af652d87825758be7c63699f712f634fb0 100644
--- a/src/x64/macro-assembler-x64.h
+++ b/src/x64/macro-assembler-x64.h
@@ -928,6 +928,11 @@ class MacroAssembler: public Assembler {
   // 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