MIPS: Fix r19360 "Improve positions tracking inside the HGraphBuilder."

BUG=
R=vegorov@chromium.org

Review URL: https://codereview.chromium.org/164033002

Patch from Balazs Kilvady <kilvadyb@homejinni.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19364 ce2b1a6d-e550-0410-aec6-3dcde31c8c00