[Interpreter] Register conversion fix and test.

test/unittests/interpreter/bytecode-array-builder-unittest.cc

 // 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.
 
 #include "src/v8.h"
 
 #include "src/interpreter/bytecode-array-builder.h"
 #include "test/unittests/test-utils.h"
 
 namespace v8 {
@@ skipping 99 matching lines @@
 
   int second;
   {
     TemporaryRegisterScope temporaries(&builder);
     second = temporaries.NewRegister().index();
   }
 
   CHECK_EQ(first, second);
 }
 
+
+TEST_F(BytecodeArrayBuilderTest, RegisterValues) {
+  int index = 1;
+  uint8_t operand = static_cast<uint8_t>(-index);
+
+  Register the_register(index);
+  CHECK_EQ(the_register.index(), index);
+
+  int actual_operand = the_register.ToOperand();
+  CHECK_EQ(actual_operand, operand);
+
+  int actual_index = Register::FromOperand(actual_operand).index();
+  CHECK_EQ(actual_index, index);
+}
+
 }  // namespace interpreter
 }  // namespace internal
 }  // namespace v8