Fix disassembly of sar(_, imm8)

R=bmeurer@chromium.org

Review URL: https://codereview.chromium.org/442003002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22902 ce2b1a6d-e550-0410-aec6-3dcde31c8c00