Reading the virtual register from a LOperand is only needed used for
unallocated LOperands (LUnallocated). There is no need for having a
method for that on LOperand.
Review URL: http://codereview.chromium.org/9293003

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@10555 ce2b1a6d-e550-0410-aec6-3dcde31c8c00