LoadCodeUnitsInstr can be passed both tagged and untagged string input.

R=vegorov@google.com
BUG=

Review URL: https://codereview.chromium.org//800833002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@42313 260f80e4-7a28-3924-810f-c04153c831b5