Drop IsPregenerated() and allow_stub_calls flag.

This also removes the fixed register hack that was required to support RecordWriteStub in the snapshot.

R=mstarzinger@chromium.org

Review URL: https://codereview.chromium.org/96853003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@18167 ce2b1a6d-e550-0410-aec6-3dcde31c8c00