MIPS: DSPr2: Added more fast-paths for OVER operation:

MIPS: DSPr2: Added more fast-paths for OVER operation:
 - over_8888_0565
 - over_n_8_8

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

lowlevel-blt-bench results

Referent (before):
        over_8888_0565 =  L1:  14.30  L2:  13.22  M: 10.43 ( 41.56%)  HT: 12.51  VT: 12.95  R: 11.82  RT:  7.34 (  49Kops/s)
            over_n_8_8 =  L1:  12.77  L2:  16.93  M: 15.03 ( 29.94%)  HT: 10.78  VT: 10.72  R: 10.29  RT:  4.92 (  33Kops/s)

Optimized:
        over_8888_0565 =  L1:  26.03  L2:  22.92  M: 15.68 ( 62.43%)  HT: 16.19  VT: 16.27  R: 14.93  RT:  8.60 (  52Kops/s)
            over_n_8_8 =  L1:  62.00  L2:  55.17  M: 40.29 ( 80.23%)  HT: 26.77  VT: 25.64  R: 24.13  RT: 10.01 (  47Kops/s)