Replace Strings with Names in load and store ICs.

Mostly mechanical, the only interesting change is in KeyedLoadIC::Load, where we now handle x[symbol] in the same way we handle x['string']

R=verwaest@chromium.org

Review URL: https://codereview.chromium.org/461603002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23038 ce2b1a6d-e550-0410-aec6-3dcde31c8c00