MIPS: DSPr2: Added several bilinear fast paths with a8 mask

MIPS: DSPr2: Added several bilinear fast paths with a8 mask

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

lowlevel-blt-bench -b

Referent (before):

  src_8888_8_8888 =  L1:   6.37  L2:   6.08  M:  5.46 ( 32.57%)  HT:  4.64  VT:  4.61  R:  4.52  RT:  2.85 (  23Kops/s)
  src_8888_8_0565 =  L1:   5.89  L2:   5.66  M:  5.11 ( 23.71%)  HT:  4.36  VT:  4.34  R:  4.26  RT:  2.71 (  22Kops/s)
  src_0565_8_x888 =  L1:   3.32  L2:   3.27  M:  3.17 ( 14.71%)  HT:  2.86  VT:  2.84  R:  2.81  RT:  2.07 (  19Kops/s)
  src_0565_8_0565 =  L1:   3.19  L2:   3.15  M:  3.05 ( 10.11%)  HT:  2.75  VT:  2.74  R:  2.71  RT:  2.00 (  18Kops/s)
 over_8888_8_8888 =  L1:   4.99  L2:   4.71  M:  4.11 ( 27.22%)  HT:  3.59  VT:  3.58  R:  3.50  RT:  2.36 (  21Kops/s)
  add_8888_8_8888 =  L1:   5.60  L2:   5.26  M:  4.52 ( 29.95%)  HT:  3.92  VT:  3.89  R:  3.80  RT:  2.49 (  21Kops/s)

Optimized:

  src_8888_8_8888 =  L1:  13.19  L2:  12.13  M:  9.75 ( 58.22%)  HT:  8.60  VT:  8.44  R:  7.90  RT:  5.06 (  33Kops/s)
  src_8888_8_0565 =  L1:  11.64  L2:  10.81  M:  9.18 ( 42.63%)  HT:  8.04  VT:  7.90  R:  7.57  RT:  5.02 (  32Kops/s)
  src_0565_8_x888 =  L1:   8.34  L2:   7.95  M:  7.29 ( 33.85%)  HT:  6.55  VT:  6.48  R:  6.25  RT:  4.35 (  30Kops/s)
  src_0565_8_0565 =  L1:   7.71  L2:   7.35  M:  6.90 ( 22.90%)  HT:  6.14  VT:  6.10  R:  5.94  RT:  4.07 (  29Kops/s)
 over_8888_8_8888 =  L1:   9.73  L2:   8.99  M:  7.15 ( 47.41%)  HT:  6.40  VT:  6.30  R:  6.11  RT:  4.28 (  30Kops/s)
  add_8888_8_8888 =  L1:  13.01  L2:  11.72  M:  8.70 ( 57.68%)  HT:  7.59  VT:  7.46  R:  7.20  RT:  4.74 (  32Kops/s)