If the input range is positive and the shift count is constant
we can replace >>> with >> to compute the output range.

For negative inputs, we can only compute a range if the 
result always fits into a signed int32.

BUG=v8:1510
Review URL: http://codereview.chromium.org/7489043

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@8735 ce2b1a6d-e550-0410-aec6-3dcde31c8c00