Fix FlooringDivByPowerOf2I on ARM/ARM64

Fix for divisor=1, found during code inspection. We can't hit this bug, due to
HDiv::Canonicalize() discarding the operation, but it might avoid a future bug
hunt. Patch also includes a small tidying of the ARM64 code.

BUG=
R=ulan@chromium.org

Review URL: https://codereview.chromium.org/280883003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21313 ce2b1a6d-e550-0410-aec6-3dcde31c8c00