MIPS: Fix failures on Octane 2.0 zlib benchmark.

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