MIPS: Fix Sunspider 1.0/crypto-aes.

* This makes sure that DoModI calculates correct result even when
  the left register is the same as the result register.
* Microoptmization: it generates smaller code when left and result
  are different registers.

TEST=benchmarks/sunspider/crypto-aes.js

BUG=

Review URL: https://codereview.chromium.org/22491002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@16085 ce2b1a6d-e550-0410-aec6-3dcde31c8c00