[turbofan][x64] Improve code generation for external reference access.

src/compiler/x64/instruction-codes-x64.h

 // Copyright 2014 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef V8_COMPILER_X64_INSTRUCTION_CODES_X64_H_
 #define V8_COMPILER_X64_INSTRUCTION_CODES_X64_H_
 
 namespace v8 {
 namespace internal {
 namespace compiler {
@@ skipping 162 matching lines @@
   V(MR2I) /* [%r1 + %r2*2 + K] */      \
   V(MR4I) /* [%r1 + %r2*3 + K] */      \
   V(MR8I) /* [%r1 + %r2*4 + K] */      \
   V(M1)   /* [      %r2*1    ] */      \
   V(M2)   /* [      %r2*2    ] */      \
   V(M4)   /* [      %r2*4    ] */      \
   V(M8)   /* [      %r2*8    ] */      \
   V(M1I)  /* [      %r2*1 + K] */      \
   V(M2I)  /* [      %r2*2 + K] */      \
   V(M4I)  /* [      %r2*4 + K] */      \
-  V(M8I)  /* [      %r2*8 + K] */
+  V(M8I)  /* [      %r2*8 + K] */      \
+  V(Root) /* [%root       + K] */
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_COMPILER_X64_INSTRUCTION_CODES_X64_H_