MIPS64: Fixed int vs. uintptr_t confusion (plus some cleanup on the way).

Port r23966 (a498d5e3)

BUG=
R=jkummerow@chromium.org, paul.lind@imgtec.com

Review URL: https://codereview.chromium.org/570563006

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23986 ce2b1a6d-e550-0410-aec6-3dcde31c8c00