Replaced unreachable code in DoStoreNamedField by an assertion.

This is part of the effort to assign an environment if and only if we can deopt.

Removed some related dead code on the way.

R=verwaest@chromium.org

Review URL: https://codereview.chromium.org/209873005

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20194 ce2b1a6d-e550-0410-aec6-3dcde31c8c00