Revert of Remove register index/code indirection

src/compiler/graph-visualizer.cc

Index: src/compiler/graph-visualizer.cc
diff --git a/src/compiler/graph-visualizer.cc b/src/compiler/graph-visualizer.cc
index 47ca259e74dc7f96f9a9faa1ef2df92f960e5e00..07ca04532bcaa3c94c95ea9d3b380d8167569f7c 100644
--- a/src/compiler/graph-visualizer.cc
+++ b/src/compiler/graph-visualizer.cc
@@ -719,13 +719,13 @@
     os_ << vreg << ":" << range->relative_id() << " " << type;
     if (range->HasRegisterAssigned()) {
       AllocatedOperand op = AllocatedOperand::cast(range->GetAssignedOperand());
+      int assigned_reg = op.index();
       if (op.IsDoubleRegister()) {
-        DoubleRegister assigned_reg = op.GetDoubleRegister();
-        os_ << " \"" << assigned_reg.ToString() << "\"";
+        os_ << " \"" << DoubleRegister::AllocationIndexToString(assigned_reg)
+            << "\"";
       } else {
         DCHECK(op.IsRegister());
-        Register assigned_reg = op.GetRegister();
-        os_ << " \"" << assigned_reg.ToString() << "\"";
+        os_ << " \"" << Register::AllocationIndexToString(assigned_reg) << "\"";
       }
     } else if (range->spilled()) {
       auto top = range->TopLevel();