Fixed flooring division by a power of 2, once again...

Avoid right shifts by zero bits: On ARM it actually means shifting by
32 bits (correctness issue) and on other platforms they are useless
(performance issue). This is fix for the fix in r20544.

BUG=v8:3259
LOG=y
R=yangguo@chromium.org

Review URL: https://codereview.chromium.org/324403003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21769 ce2b1a6d-e550-0410-aec6-3dcde31c8c00