The input register does not need to be made writable. We already
allocate enough temp registers and the input register will always
be preserved.
Review URL: http://codereview.chromium.org/9166007

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@10379 ce2b1a6d-e550-0410-aec6-3dcde31c8c00