Handle HCheckInstanceType and HIsStringAndBranch in check elimination.

R=ishell@chromium.org

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

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21593 ce2b1a6d-e550-0410-aec6-3dcde31c8c00