Turn keyed loads with string-based (non-convertible to array-index) key into named loads

BUG=
R=ishell@chromium.org

Review URL: https://codereview.chromium.org/585433002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24032 ce2b1a6d-e550-0410-aec6-3dcde31c8c00