arm: Fix Smi check in SubStringStub::Generate
arm: Fix Smi check in SubStringStub::Generate

R=ulan@chromium.org

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

git-svn-id: b158db1e4b4ab85d4c9e510fdef4b1e8c614b15b@21050 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh