Optimize register constaints for NumberTagU Lithium instruction, and adjust code
generated to match.

TEST=none
BUG=
R=ulan@chromium.org

Review URL: https://codereview.chromium.org/94983002

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@18162 ce2b1a6d-e550-0410-aec6-3dcde31c8c00