MIPS: Re-work DoModI.

MIPS: Re-work DoModI.

Changes:
-separated the two code-paths (constant power of 2 divisor similar to ARM and everything else)
-replaced a bailout condition with faster handling (negative modulo result)
-removed a possibly useless mov instruction from one path
-replaced the IsConstantOperand condition with the more meaningful HasPowerOf2Divisor (although in this specific case they're equivalent)

BUG=
TEST=

Review URL: http://codereview.chromium.org/8591002
Patch from Gergely Kis <gergely@homejinni.com>.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@10016 ce2b1a6d-e550-0410-aec6-3dcde31c8c00