arm: Fix Smi check in SubStringStub::Generate

R=ulan@chromium.org

Review URL: https://codereview.chromium.org/256743007

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21050 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
Object::Lookup(), JSObject::*Lookup*() and JSReceiver::*Lookup*() handlified.

R=yangguo@chromium.org

Review URL: https://codereview.chromium.org/253843006

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21051 ce2b1a6d-e550-0410-aec6-3dcde31c8c00