MIPS: Let NaN flow as double into HBranch + some minor improvements

Port r15246 (cb18dce2)

BUG=
R=plind44@gmail.com

Review URL: https://codereview.chromium.org/17198012

Patch from Akos Palfi <palfia@homejinni.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@15259 ce2b1a6d-e550-0410-aec6-3dcde31c8c00