- Added a general DoubleToIStub so that it's possible to extend to other platforms and non-truncating case.
- This version handles all cases of truncation (previous code deopted in some cases) and all source/destination register combinations without clobbering any temps.

R=yangguo@chromium.org

Review URL: https://codereview.chromium.org/18612005

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@15645 ce2b1a6d-e550-0410-aec6-3dcde31c8c00