MIPS: DSPr2: Added several nearest neighbor fast paths with a8 mask:

MIPS: DSPr2: Added several nearest neighbor fast paths with a8 mask:

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

lowlevel-blt-bench -n

Referent (before):
        over_8888_8_0565 =  L1:   9.62  L2:   8.85  M:  7.40 ( 39.27%)  HT:  5.67  VT:  5.61  R:  5.45  RT:  2.98 (  22Kops/s)
        over_0565_8_0565 =  L1:   7.90  L2:   7.49  M:  6.72 ( 26.75%)  HT:  5.24  VT:  5.20  R:  5.06  RT:  2.90 (  22Kops/s)

Optimized:
        over_8888_8_0565 =  L1:  18.51  L2:  16.82  M: 12.13 ( 64.43%)  HT: 10.06  VT:  9.88  R:  9.54  RT:  5.63 (  31Kops/s)
        over_0565_8_0565 =  L1:  14.82  L2:  13.94  M: 11.34 ( 45.20%)  HT:  9.45  VT:  9.35  R:  9.03  RT:  5.50 (  31Kops/s)