Make register allocator to disregard constant computation with no uses.

runtime/vm/assembler_x64.h

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #ifndef VM_ASSEMBLER_X64_H_
 #define VM_ASSEMBLER_X64_H_
 
 #ifndef VM_ASSEMBLER_H_
 #error Do not include assembler_x64.h directly; use assembler.h instead.
 #endif
@@ skipping 374 matching lines @@
   void andq(Register dst, const Address& address);
   void andq(Register dst, const Immediate& imm);
 
   void orq(Register dst, Register src);
   void orq(Register dst, const Address& address);
   void orq(Register dst, const Immediate& imm);
 
   void xorq(Register dst, Register src);
   void xorq(Register dst, const Address& address);
   void xorq(const Address& dst, Register src);
+  void xorq(Register dst, const Immediate& imm);
 
   void addl(Register dst, Register src);
   void addl(const Address& address, const Immediate& imm);
 
   void addq(Register dst, Register src);
   void addq(Register reg, const Immediate& imm);
   void addq(Register reg, const Address& address);
   void addq(const Address& address, const Immediate& imm);
   void addq(const Address& address, Register reg);
 
@@ skipping 299 matching lines @@
 }
 
 
 inline void Assembler::EmitOperandSizeOverride() {
   EmitUint8(0x66);
 }
 
 }  // namespace dart
 
 #endif  // VM_ASSEMBLER_X64_H_