Adds the x86-64 assembler.

src/IceRegistersX8664.h

 //===- subzero/src/IceRegistersX8664.h - Register information ---*- C++ -*-===//
 //
 //                        The Subzero Code Generator
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 ///
 /// \file
@@ skipping 61 matching lines @@
     assert(Reg_GPR_First <= RegNum && RegNum <= Reg_GPR_Last);
     return GPRRegister(RegNum - Reg_GPR_First);
   }
 
   static inline XmmRegister getEncodedXmm(int32_t RegNum) {
     assert(Reg_XMM_First <= RegNum && RegNum <= Reg_XMM_Last);
     return XmmRegister(RegNum - Reg_XMM_First);
   }
 
   static inline ByteRegister getEncodedByteReg(int32_t RegNum) {
-    // In x86-64, AH is not encodable when the REX prefix is used; the same
-    // encoding is used for spl. Therefore, ah needs special handling.
-    if (RegNum == Reg_ah)
-      return Encoded_Reg_spl;
     return ByteRegister(RegNum - Reg_GPR_First);
   }
 
   static inline GPRRegister getEncodedByteRegOrGPR(Type Ty, int32_t RegNum) {
     if (isByteSizedType(Ty))
       return GPRRegister(getEncodedByteReg(RegNum));
     else
       return getEncodedGPR(RegNum);
   }
 };
 
 } // end of namespace Ice
 
 #endif // SUBZERO_SRC_ICEREGISTERSX8664_H