Refactored DirectCEntryStub::GenerateCall a bit to make it clearer what's going on and added an ASSERT.
Review URL: http://codereview.chromium.org/7640016

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@8938 ce2b1a6d-e550-0410-aec6-3dcde31c8c00