fix imul(reg, op, imm) emission on x64

R=bmeurer@chromium.org

BUG=

Review URL: https://codereview.chromium.org/619903005

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24395 ce2b1a6d-e550-0410-aec6-3dcde31c8c00