Handle HCheckInstanceType and HIsStringAndBranch in check elimination.

R=ishell@chromium.org

Committed: https://code.google.com/p/v8/source/detail?r=21593

Review URL: https://codereview.chromium.org/300423003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21606 ce2b1a6d-e550-0410-aec6-3dcde31c8c00