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

src/compiler/x64/code-generator-x64.cc

 // Copyright 2013 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.
 
 #include "src/compiler/code-generator.h"
 
 #include "src/compilation-info.h"
 #include "src/compiler/code-generator-impl.h"
 #include "src/compiler/gap-resolver.h"
 #include "src/compiler/node-matchers.h"
@@ skipping 115 matching lines @@
       }
       case kMode_M1I:
       case kMode_M2I:
       case kMode_M4I:
       case kMode_M8I: {
         Register index = InputRegister(NextOffset(offset));
         ScaleFactor scale = ScaleFor(kMode_M1I, mode);
         int32_t disp = InputInt32(NextOffset(offset));
         return Operand(index, scale, disp);
       }
+      case kMode_Root: {
+        Register base = kRootRegister;
+        int32_t disp = InputInt32(NextOffset(offset));
+        return Operand(base, disp);
+      }
       case kMode_None:
         UNREACHABLE();
         return Operand(no_reg, 0);
     }
     UNREACHABLE();
     return Operand(no_reg, 0);
   }
 
   Operand MemoryOperand(size_t first_input = 0) {
     return MemoryOperand(&first_input);
@@ skipping 2569 matching lines @@
     int padding_size = last_lazy_deopt_pc_ + space_needed - current_pc;
     __ Nop(padding_size);
   }
 }
 
 #undef __
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8