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

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

Index: src/compiler/x64/code-generator-x64.cc
diff --git a/src/compiler/x64/code-generator-x64.cc b/src/compiler/x64/code-generator-x64.cc
index 56cd3ea0258e7068c359d20b2141a4539ff04451..4d63e9ad8355479b6f2f95d2351b9b91df14acf5 100644
--- a/src/compiler/x64/code-generator-x64.cc
+++ b/src/compiler/x64/code-generator-x64.cc
@@ -133,6 +133,11 @@ class X64OperandConverter : public InstructionOperandConverter {
         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);