MIPS: DSPr2: Added OVER combiner and two new fast paths: - over_8888_8888 - over_8888...

MIPS: DSPr2: Added OVER combiner and two new fast paths: - over_8888_8888 - over_8888_8888_8888

Performance numbers before/after on MIPS-74kc @ 1GHz:

lowlevel-blt-bench results

Referent (before):
          over_8888_8888 =  L1:  19.61  L2:  17.10  M: 11.16 ( 59.20%)  HT: 16.47  VT: 15.81  R: 14.82  RT:  8.90 (  50Kops/s)
     over_8888_8888_8888 =  L1:  13.56  L2:  11.22  M:  7.46 ( 79.18%)  HT:  6.24  VT:  6.20  R:  6.11  RT:  3.95 (  29Kops/s)

Optimized:
          over_8888_8888 =  L1:  46.42  L2:  36.70  M: 16.69 ( 88.57%)  HT: 17.11  VT: 16.55  R: 15.31  RT:  9.48 (  52Kops/s)
     over_8888_8888_8888 =  L1:  26.06  L2:  22.53  M: 11.49 (121.91%)  HT:  9.93  VT:  9.62  R:  9.19  RT:  5.75 (  36Kops/s)