Handle flooring division in LCodeGen::DoDivByConstI on ARM, too.

We should really split up the Lithium instruction, but this will be
done in some future cleanup CL.

Removed some "const"s for local variables on the way, they don't really
help us much and just clutter up the code.

R=ulan@chromium.org

Review URL: https://codereview.chromium.org/196603004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19850 ce2b1a6d-e550-0410-aec6-3dcde31c8c00