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

Kendryte K230                Before             After         Delta

blend_w4_16bpc_c:         227.0 ( 1.00x)    227.1 ( 1.00x)    0.04%
blend_w4_16bpc_rvv:        71.1 ( 3.19x)     73.2 ( 3.10x)    2.95%
blend_w8_16bpc_c:         662.5 ( 1.00x)    662.7 ( 1.00x)    0.03%
blend_w8_16bpc_rvv:       132.4 ( 5.00x)    115.0 ( 5.76x)  -13.14%
blend_w16_16bpc_c:       2559.3 ( 1.00x)   2559.8 ( 1.00x)    0.02%
blend_w16_16bpc_rvv:      416.1 ( 6.15x)    326.7 ( 7.83x)  -21.49%
blend_w32_16bpc_c:       6483.9 ( 1.00x)   6484.5 ( 1.00x)    0.01%
blend_w32_16bpc_rvv:     1029.1 ( 6.30x)    774.7 ( 8.37x)  -24.72%

SpacemiT K1                  Before             After         Delta

blend_w4_16bpc_c:         206.1 ( 1.00x)    207.0 ( 1.00x)    0.44%
blend_w4_16bpc_rvv:        64.4 ( 3.20x)     69.5 ( 2.98x)    7.92%
blend_w8_16bpc_c:         600.2 ( 1.00x)    600.9 ( 1.00x)    0.12%
blend_w8_16bpc_rvv:       101.6 ( 5.91x)    106.9 ( 5.62x)    5.22%
blend_w16_16bpc_c:       2316.0 ( 1.00x)   2316.4 ( 1.00x)    0.02%
blend_w16_16bpc_rvv:      261.8 ( 8.85x)    229.1 (10.11x)  -12.49%
blend_w32_16bpc_c:       5861.1 ( 1.00x)   5860.4 ( 1.00x)   -0.01%
blend_w32_16bpc_rvv:      602.9 ( 9.72x)    475.3 (12.33x)  -21.16%

Blackhole p100a              Before             After         Delta

blend_w4_16bpc_c:         193.3 ( 1.00x)    191.3 ( 1.00x)   -1.03%
blend_w4_16bpc_rvv:        66.3 ( 2.91x)     65.4 ( 2.92x)   -1.36%
blend_w8_16bpc_c:         552.0 ( 1.00x)    549.8 ( 1.00x)   -0.40%
blend_w8_16bpc_rvv:       100.5 ( 5.49x)     96.2 ( 5.71x)   -4.28%
blend_w16_16bpc_c:       2112.5 ( 1.00x)   2111.8 ( 1.00x)   -0.03%
blend_w16_16bpc_rvv:      190.3 (11.10x)    185.9 (11.36x)   -2.31%
blend_w32_16bpc_c:       5417.5 ( 1.00x)   5416.2 ( 1.00x)   -0.02%
blend_w32_16bpc_rvv:      290.3 (18.66x)    304.0 (17.82x)    4.72%