Using rsp or r12 as the "base" register of the ModR/M byte forces a SIB byte,
even with no index register. Some operations can avoid this by using another,
equivalent, encoding that swaps the meaning of the base and register parts.

Review URL: http://codereview.chromium.org/2075010

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@4678 ce2b1a6d-e550-0410-aec6-3dcde31c8c00