MIPS: added optimizations for functions from SkBitmapProcState

gain is ~30%

following functions are optimized:
  SI8_D16_nofilter_DX
  SI8_opaque_D32_nofilter_DX

R=djsollen@google.com, teodora.petrovic@gmail.com

Author: djordje.pesut@imgtec.com

Review URL: https://codereview.chromium.org/336533003
2 files changed