Reland and fix "Fix LoadFieldByIndex to take mutable heap-numbers into account.""

BUG=
R=hpayer@chromium.org

Review URL: https://codereview.chromium.org/218663005

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20358 ce2b1a6d-e550-0410-aec6-3dcde31c8c00