Fix bug in disassembler output (using non-generic registers for memory operands in some cases).
Fix bug in movsxbq and make REX optional in movzx?q.

Review URL: http://codereview.chromium.org/2834027

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@4977 ce2b1a6d-e550-0410-aec6-3dcde31c8c00