BFC requires same register for input and output and causes introduction
of some unneccesary gap moves.

TEST=unittests,mjsunit
R=svenpanne@chromium.org

Review URL: https://codereview.chromium.org/658283003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24678 ce2b1a6d-e550-0410-aec6-3dcde31c8c00