X64 implementation starts using virtual frame and register allocators.

src/x64/assembler-x64.h

Index: src/x64/assembler-x64.h
===================================================================
--- src/x64/assembler-x64.h	(revision 2145)
+++ src/x64/assembler-x64.h	(working copy)
@@ -482,6 +482,10 @@
     immediate_arithmetic_op_32(0x0, dst, src);
   }
 
+  void cmpb(const Operand& dst, Immediate src) {
+    immediate_arithmetic_op_8(0x7, dst, src);
+  }
+
   void cmpq(Register dst, Register src) {
     arithmetic_op(0x3B, dst, src);
   }
@@ -958,9 +962,14 @@
   void arithmetic_op(byte opcode, Register reg, const Operand& op);
   void immediate_arithmetic_op(byte subcode, Register dst, Immediate src);
   void immediate_arithmetic_op(byte subcode, const Operand& dst, Immediate src);
+  // Operate on a 32-bit word in memory.
   void immediate_arithmetic_op_32(byte subcode,
                                   const Operand& dst,
                                   Immediate src);
+  // Operate on a byte in memory.
+  void immediate_arithmetic_op_8(byte subcode,
+                                  const Operand& dst,
+                                  Immediate src);
   // Emit machine code for a shift operation.
   void shift(Register dst, Immediate shift_amount, int subcode);
   // Shift dst by cl % 64 bits.