MIPS: DSPr2: Added fast-paths for OVER operation: - over_8888_n_0565 - over_8888_8_0565

MIPS: DSPr2: Added fast-paths for OVER operation: - over_8888_n_0565 - over_8888_8_0565

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

lowlevel-blt-bench results

Referent (before):
        over_8888_n_0565 =  L1:   8.95  L2:   8.33  M:  6.95 ( 27.74%)  HT:  4.27  VT:  4.07  R:  4.01  RT:  1.74 (  19Kops/s)
        over_8888_8_0565 =  L1:   8.86  L2:   8.11  M:  6.72 ( 35.71%)  HT:  5.68  VT:  5.62  R:  5.47  RT:  3.35 (  30Kops/s)

Optimized:
        over_8888_n_0565 =  L1:  18.76  L2:  17.55  M: 13.11 ( 52.19%)  HT: 11.35  VT: 11.10  R: 10.88  RT:  6.94 (  47Kops/s)
        over_8888_8_0565 =  L1:  18.14  L2:  16.79  M: 12.10 ( 64.25%)  HT: 10.24  VT:  9.98  R:  9.63  RT:  5.89 (  43Kops/s)