Use %_IsSmi instead of %MaxSmi.

ToPositiveInteger already makes sure that if l can be represented as a
smi, it will be represented as a smi.  This way we can avoid doing a
runtime call to retrieve a constant.

R=dslomov@chromium.org

Review URL: https://codereview.chromium.org/81063002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@17983 ce2b1a6d-e550-0410-aec6-3dcde31c8c00