Move ToInt conversions to the MacroAssembler for x64

+ Prevent truncating TaggedToI from bailing out.

(This is a port of r16464)

BUG=
R=titzer@chromium.org

Review URL: https://codereview.chromium.org/23938003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@16615 ce2b1a6d-e550-0410-aec6-3dcde31c8c00