Replace LoadNamedFieldPolymorphic with explicit branches.

R=ulan@chromium.org

Review URL: https://chromiumcodereview.appspot.com/22213002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@16133 ce2b1a6d-e550-0410-aec6-3dcde31c8c00