MIPS: Only assign environments when they are actually needed.

Port r20376 (2c4baaf5)

BUG=
R=plind44@gmail.com

Review URL: https://codereview.chromium.org/213733003

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@20416 ce2b1a6d-e550-0410-aec6-3dcde31c8c00