MIPS: Drop IsPregenerated() and allow_stub_calls flag.

Port r18167 (1e4b11e0)

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

BUG=
R=plind44@gmail.com

Review URL: https://codereview.chromium.org/97373002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@18169 ce2b1a6d-e550-0410-aec6-3dcde31c8c00