Lithium: avoid registers for constants when possible.

BUG=

Review URL: https://codereview.chromium.org/14364010

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@14460 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
remove IsOneByteConvertible

R=ulan@chromium.org
BUG=

Review URL: https://codereview.chromium.org/14298021

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@14461 ce2b1a6d-e550-0410-aec6-3dcde31c8c00