Currently we can't specify a set of registers as a register constraint.
This change forces a fixed register (eax) in places that need
a byte register.

BUG=77752
Review URL: http://codereview.chromium.org/6708109

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@7417 ce2b1a6d-e550-0410-aec6-3dcde31c8c00