The API is inconsistent about when a register must be coerced to an operand
and when it can be used as a register.  Simplify usage by never requiring it
to be wrapped.

R=fschneider@chromium.org
BUG=
TEST=

Review URL: http://codereview.chromium.org/8086021

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@9507 ce2b1a6d-e550-0410-aec6-3dcde31c8c00