Fix awesomeness in X64 assembler.

R=ulan@chromium.org
TEST=mjsunit --turbo-types

Review URL: https://codereview.chromium.org/559123003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23844 ce2b1a6d-e550-0410-aec6-3dcde31c8c00