Add support to load/store byte fields.

This adds a new Byte representation and support for zero-extended
loads in HLoadNamedField and truncated stores in HStoreNamedField.

R=mvstanton@chromium.org

Committed: https://code.google.com/p/v8/source/detail?r=17079

Review URL: https://codereview.chromium.org/25696004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@17100 ce2b1a6d-e550-0410-aec6-3dcde31c8c00