Revert of Remove register index/code indirection

src/compiler/code-generator-impl.h

Index: src/compiler/code-generator-impl.h
diff --git a/src/compiler/code-generator-impl.h b/src/compiler/code-generator-impl.h
index df33f434b1ef6c6c1ca75d146bd005bd45b5b38f..f0762e9bbec2e0657c6e8372e643c0eed4db47d0 100644
--- a/src/compiler/code-generator-impl.h
+++ b/src/compiler/code-generator-impl.h
@@ -94,11 +94,12 @@
   }
 
   Register ToRegister(InstructionOperand* op) {
-    return RegisterOperand::cast(op)->GetRegister();
+    return Register::FromAllocationIndex(RegisterOperand::cast(op)->index());
   }
 
   DoubleRegister ToDoubleRegister(InstructionOperand* op) {
-    return DoubleRegisterOperand::cast(op)->GetDoubleRegister();
+    return DoubleRegister::FromAllocationIndex(
+        DoubleRegisterOperand::cast(op)->index());
   }
 
   Constant ToConstant(InstructionOperand* op) {