MIPS: Fix FlooringDivByPowerOf2I.

Port r21313 (3b4cb0b)

Original commit message:
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=plind44@gmail.com

Review URL: https://codereview.chromium.org/277893003

Patch from Balazs Kilvady <kilvadyb@homejinni.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21317 ce2b1a6d-e550-0410-aec6-3dcde31c8c00