Set kTruncatingToSmi for HChange only when To rep is Smi

BUG=
R=verwaest@chromium.org

Review URL: https://codereview.chromium.org/218613002

Patch from Weiliang Lin <weiliang.lin@intel.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20385 ce2b1a6d-e550-0410-aec6-3dcde31c8c00