riscv64/mc: Only process w*3/4 elements in blend_v

Setting VL for this function only impacts the 16bpc performance and only
 on the SpacemiT K1 which has two vector units of length 128b each.

Kendryte K230                Before             After         Delta

blend_v_w2_8bpc_c:        220.0 ( 1.00x)    221.3 ( 1.00x)    0.59%
blend_v_w2_8bpc_rvv:      145.7 ( 1.51x)    148.2 ( 1.49x)    1.72%
blend_v_w4_8bpc_c:        942.1 ( 1.00x)    943.7 ( 1.00x)    0.17%
blend_v_w4_8bpc_rvv:      240.4 ( 3.92x)    242.9 ( 3.89x)    1.04%
blend_v_w8_8bpc_c:       1782.3 ( 1.00x)   1783.8 ( 1.00x)    0.08%
blend_v_w8_8bpc_rvv:      252.6 ( 7.06x)    254.9 ( 7.00x)    0.91%
blend_v_w16_8bpc_c:      3650.9 ( 1.00x)   3647.0 ( 1.00x)   -0.11%
blend_v_w16_8bpc_rvv:     495.5 ( 7.37x)    494.4 ( 7.38x)   -0.22%
blend_v_w32_8bpc_c:      7013.0 ( 1.00x)   7018.2 ( 1.00x)    0.07%
blend_v_w32_8bpc_rvv:     807.9 ( 8.68x)    802.0 ( 8.75x)   -0.73%

blend_v_w2_16bpc_c:       226.1 ( 1.00x)    225.5 ( 1.00x)   -0.27%
blend_v_w2_16bpc_rvv:     148.6 ( 1.52x)    148.9 ( 1.51x)    0.20%
blend_v_w4_16bpc_c:      1010.7 ( 1.00x)   1006.7 ( 1.00x)   -0.40%
blend_v_w4_16bpc_rvv:     306.7 ( 3.30x)    307.4 ( 3.27x)    0.23%
blend_v_w8_16bpc_c:      1990.2 ( 1.00x)   1996.1 ( 1.00x)    0.30%
blend_v_w8_16bpc_rvv:     519.5 ( 3.83x)    523.4 ( 3.81x)    0.75%
blend_v_w16_16bpc_c:     3744.5 ( 1.00x)   3742.4 ( 1.00x)   -0.06%
blend_v_w16_16bpc_rvv:    899.6 ( 4.16x)    906.4 ( 4.13x)    0.76%
blend_v_w32_16bpc_c:     7047.5 ( 1.00x)   7079.3 ( 1.00x)    0.45%
blend_v_w32_16bpc_rvv:   1475.5 ( 4.78x)   1483.3 ( 4.77x)    0.53%

SpacemiT K1                  Before             After         Delta

blend_v_w2_8bpc_c:        216.3 ( 1.00x)    214.4 ( 1.00x)   -0.88%
blend_v_w2_8bpc_rvv:      144.0 ( 1.50x)    143.6 ( 1.49x)   -0.28%
blend_v_w4_8bpc_c:        919.8 ( 1.00x)    918.1 ( 1.00x)   -0.18%
blend_v_w4_8bpc_rvv:      236.6 ( 3.89x)    236.4 ( 3.88x)   -0.08%
blend_v_w8_8bpc_c:       1739.3 ( 1.00x)   1736.8 ( 1.00x)   -0.14%
blend_v_w8_8bpc_rvv:      236.8 ( 7.34x)    236.3 ( 7.35x)   -0.21%
blend_v_w16_8bpc_c:      3374.7 ( 1.00x)   3374.9 ( 1.00x)    0.01%
blend_v_w16_8bpc_rvv:     297.0 (11.36x)    296.8 (11.37x)   -0.07%
blend_v_w32_8bpc_c:      6647.5 ( 1.00x)   6645.5 ( 1.00x)   -0.03%
blend_v_w32_8bpc_rvv:     403.3 (16.48x)    402.4 (16.51x)   -0.22%

blend_v_w2_16bpc_c:       221.4 ( 1.00x)    220.1 ( 1.00x)   -0.59%
blend_v_w2_16bpc_rvv:     146.3 ( 1.51x)    147.3 ( 1.49x)    0.68%
blend_v_w4_16bpc_c:       973.3 ( 1.00x)    972.7 ( 1.00x)   -0.06%
blend_v_w4_16bpc_rvv:     280.3 ( 3.47x)    282.1 ( 3.45x)    0.64%
blend_v_w8_16bpc_c:      1814.8 ( 1.00x)   1816.2 ( 1.00x)    0.08%
blend_v_w8_16bpc_rvv:     376.6 ( 4.82x)    376.9 ( 4.82x)    0.08%
blend_v_w16_16bpc_c:     3485.5 ( 1.00x)   3485.5 ( 1.00x)    0.00%
blend_v_w16_16bpc_rvv:    531.1 ( 6.56x)    525.6 ( 6.63x)   -1.04%
blend_v_w32_16bpc_c:     6788.3 ( 1.00x)   6778.8 ( 1.00x)   -0.14%
blend_v_w32_16bpc_rvv:    904.5 ( 7.51x)    854.6 ( 7.93x)   -5.52%