Revert "Handle HCheckInstanceType and HIsStringAndBranch in check elimination."

This reverts commit r21593 for breaking the Webkit tests.

TBR=mvstanton@chromium.org

Review URL: https://codereview.chromium.org/310593004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21594 ce2b1a6d-e550-0410-aec6-3dcde31c8c00