Make HForceRepresentation an idef.

An optimization in the array constructor wasn't firing because the code
would check for constant values, failing to find them through
intervening HForceRepresentation instructions. Repaired this site. A
follow-on CL will provide a better way of making decisions based on
constant HValues.

R=verwaest@chromium.org

Review URL: https://codereview.chromium.org/59623009

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@17708 ce2b1a6d-e550-0410-aec6-3dcde31c8c00