The bug was introduced in af9504e.

The zlib benchmark triggers long branch mode on MIPS, in which
at register is clobbered after every branch.

TEST=benchmarks/octane/zlib
BUG=
R=plind44@gmail.com

Review URL: https://codereview.chromium.org/95203005

Patch from Dusan Milosavljevic <Dusan.Milosavljevic@rt-rk.com>.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@18168 ce2b1a6d-e550-0410-aec6-3dcde31c8c00