X64: Reapply fix for allocating HeapNumber results for >>>. Also skip case where both number arguments are smis.
X64: Reapply fix for allocating HeapNumber results for >>>.
Also skip case where both number arguments are smis.

Review URL: http://codereview.chromium.org/2814021

git-svn-id: b158db1e4b4ab85d4c9e510fdef4b1e8c614b15b@4905 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

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