Lower simplified StringEqual to runtime call.

R=titzer@chromium.org
TEST=cctest/test-simplified-lowering/LowerStringOps_to_call_and_wordeq

Review URL: https://codereview.chromium.org/531763002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23616 ce2b1a6d-e550-0410-aec6-3dcde31c8c00