riscv64/mc16: Keep blend_v RVV operations in 16-bits

Kendryte K230                Before             After         Delta

blend_v_w2_16bpc_c:       240.9 ( 1.00x)    240.9 ( 1.00x)    0.00%
blend_v_w2_16bpc_rvv:     149.7 ( 1.61x)    155.4 ( 1.55x)    3.81%
blend_v_w4_16bpc_c:      1072.4 ( 1.00x)   1072.5 ( 1.00x)    0.01%
blend_v_w4_16bpc_rvv:     307.2 ( 3.49x)    299.9 ( 3.58x)   -2.38%
blend_v_w8_16bpc_c:      2004.7 ( 1.00x)   2010.2 ( 1.00x)    0.27%
blend_v_w8_16bpc_rvv:     436.1 ( 4.60x)    381.0 ( 5.28x)  -12.63%
blend_v_w16_16bpc_c:     3859.4 ( 1.00x)   3853.7 ( 1.00x)   -0.15%
blend_v_w16_16bpc_rvv:    761.1 ( 5.07x)    554.0 ( 6.96x)  -27.21%
blend_v_w32_16bpc_c:     7509.7 ( 1.00x)   7505.3 ( 1.00x)   -0.06%
blend_v_w32_16bpc_rvv:   1427.1 ( 5.26x)   1005.5 ( 7.46x)  -29.54%

SpacemiT K1                  Before             After         Delta

blend_v_w2_16bpc_c:       220.1 ( 1.00x)    222.0 ( 1.00x)    0.86%
blend_v_w2_16bpc_rvv:     146.6 ( 1.50x)    151.1 ( 1.47x)    3.07%
blend_v_w4_16bpc_c:       968.3 ( 1.00x)    969.6 ( 1.00x)    0.13%
blend_v_w4_16bpc_rvv:     281.2 ( 3.44x)    290.2 ( 3.34x)    3.20%
blend_v_w8_16bpc_c:      1809.5 ( 1.00x)   1812.1 ( 1.00x)    0.14%
blend_v_w8_16bpc_rvv:     374.2 ( 4.84x)    375.3 ( 4.83x)    0.29%
blend_v_w16_16bpc_c:     3479.7 ( 1.00x)   3480.9 ( 1.00x)    0.03%
blend_v_w16_16bpc_rvv:    521.5 ( 6.67x)    465.9 ( 7.47x)  -10.66%
blend_v_w32_16bpc_c:     6767.9 ( 1.00x)   6773.7 ( 1.00x)    0.09%
blend_v_w32_16bpc_rvv:    852.1 ( 7.94x)    727.4 ( 9.31x)  -14.63%

Blackhole p100a              Before             After         Delta

blend_v_w2_16bpc_c:       205.6 ( 1.00x)    206.0 ( 1.00x)    0.19%
blend_v_w2_16bpc_rvv:     176.5 ( 1.16x)    143.6 ( 1.44x)  -18.64%
blend_v_w4_16bpc_c:       901.0 ( 1.00x)    891.8 ( 1.00x)   -1.02%
blend_v_w4_16bpc_rvv:     298.8 ( 3.02x)    235.2 ( 3.79x)  -21.29%
blend_v_w8_16bpc_c:      1663.3 ( 1.00x)   1656.5 ( 1.00x)   -0.41%
blend_v_w8_16bpc_rvv:     300.1 ( 5.54x)    236.4 ( 7.01x)  -21.23%
blend_v_w16_16bpc_c:     3192.1 ( 1.00x)   3182.3 ( 1.00x)   -0.31%
blend_v_w16_16bpc_rvv:    349.2 ( 9.14x)    311.4 (10.22x)  -10.82%
blend_v_w32_16bpc_c:     6259.2 ( 1.00x)   6257.8 ( 1.00x)   -0.02%
blend_v_w32_16bpc_rvv:    350.2 (17.88x)    321.8 (19.44x)   -8.11%