Optimize register constraints and code generated for Integer32ToSmi Lithium
instruction.

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

Review URL: https://codereview.chromium.org/88383002

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@18084 ce2b1a6d-e550-0410-aec6-3dcde31c8c00