MIPS: DSPr2: Added over_n_8_8888 and over_n_8_0565 fast paths.

MIPS: DSPr2: Added over_n_8_8888 and over_n_8_0565 fast paths.

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

Referent (before):

lowlevel-blt-bench:
     over_n_8_8888 =  L1:  10.40  L2:   9.79  M:  8.47 ( 33.62%)  HT:  7.64  VT:  7.59  R:  7.48  RT:  5.30 (  40Kops/s)
     over_n_8_0565 =  L1:   7.40  L2:   7.23  M:  6.78 ( 17.94%)  HT:  6.23  VT:  6.17  R:  6.14  RT:  4.62 (  37Kops/s)

Optimized:

lowlevel-blt-bench:
     over_n_8_8888 =  L1:  27.25  L2:  26.24  M: 18.15 ( 72.12%)  HT: 14.52  VT: 14.31  R: 13.83  RT:  7.57 (  48Kops/s)
     over_n_8_0565 =  L1:  18.91  L2:  17.59  M: 15.06 ( 39.90%)  HT: 12.18  VT: 11.98  R: 11.83  RT:  6.80 (  46Kops/s)