Merged r16200 into trunk branch.

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.

R=danno@chromium.org
BUG=

Review URL: https://codereview.chromium.org/23026005

git-svn-id: https://v8.googlecode.com/svn/trunk@16201 ce2b1a6d-e550-0410-aec6-3dcde31c8c00