Improvements in lithium code generation. Recognizing if some operands are constants, we can often save on registers and instructions.

BUG=

Review URL: https://codereview.chromium.org/14022011

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@14364 ce2b1a6d-e550-0410-aec6-3dcde31c8c00