The class did not correctly implement the RequiredInputRepresentation.
I changed this functions to be abstract so that all hydrogen classes
must implement it.

As a convention instructions with zero input operands return None as input
representation.

Instructions that can handle all input representations without converting before
also have None as required input representation (e.g. HTest)

All other instructions need a proper required input representation.

Review URL: http://codereview.chromium.org/6538088

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@6891 ce2b1a6d-e550-0410-aec6-3dcde31c8c00