[Interpreter] Fix incorrect Register OperandSize calculation for ExtraWide.

test/mjsunit/regress/regress-605470.js

+// Copyright 2016 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.
+
+// Flags: --ignition
+
+function function_with_m_args(m) {
+  var source = '(function f() { return; })(';
+  for (var arg = 0; arg < m ; arg++) {
+    if (arg != 0) source += ',';
+    source += arg;
+  }
+  source += ')';
+  return eval(source);
+}
+
+function_with_m_args(0x7FFF);