Arithmetic right shifting is *not* division in two's complement
representation, only in one's complement. So we convert to one's
complement, shift, and go back to two's complement. By permutating the
last steps, one can get efficient branch-free code. This insight comes
from the paleozoic era of computer science, see the paper from 1976:

   Guy Lewis Steele Jr.: "Arithmetic Shifting Considered Harmful"
   ftp://publications.ai.mit.edu/ai-publications/pdf/AIM-378.pdf

This results in better and more correct code than our previous
"neg/shift/neg" dance.

LOG=y
BUG=v8:3151
R=bmeurer@chromium.org

Review URL: https://codereview.chromium.org/166793002

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@19434 ce2b1a6d-e550-0410-aec6-3dcde31c8c00