Fix HObjectAccess for loads from migrating prototypes

BUG=chromium:305309
R=danno@chromium.org

Review URL: https://codereview.chromium.org/35173005

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@17345 ce2b1a6d-e550-0410-aec6-3dcde31c8c00