MIPS: Fixed flooring division by -1.

Port r20544 (a64196c)

Original commit message:
We should avoid ASR #0 on ARM. Simplified and improved code a bit
while we're there on all platforms. Human brains are very bad at
understanding nested structures...

BUG=v8:3259
LOG=y
R=plind44@gmail.com

Review URL: https://codereview.chromium.org/227583003

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

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20553 ce2b1a6d-e550-0410-aec6-3dcde31c8c00