Add HForceRepresentation, to represent the implicit ToNumber applied to the input of a count operation.

BUG=v8:1389

TEST=

Review URL: http://codereview.chromium.org/7033008

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@7913 ce2b1a6d-e550-0410-aec6-3dcde31c8c00