MIPS: Never hchange nan-hole to hole or hole to nan-hole. Only allow changing hole to nan if all uses allow undefined as nan.

Port r16183 (1858bc57)

BUG=

Review URL: https://codereview.chromium.org/23130006

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@16200 ce2b1a6d-e550-0410-aec6-3dcde31c8c00