riscv64/mc: Branchless vsetvl in blend_v function

Kendryte K230

blend_v_w2_8bpc_c:       221.4 ( 1.00x)
blend_v_w2_8bpc_rvv:     147.7 ( 1.50x)
blend_v_w4_8bpc_c:       945.3 ( 1.00x)
blend_v_w4_8bpc_rvv:     243.3 ( 3.89x)
blend_v_w8_8bpc_c:      1786.9 ( 1.00x)
blend_v_w8_8bpc_rvv:     256.1 ( 6.98x)
blend_v_w16_8bpc_c:     3472.1 ( 1.00x)
blend_v_w16_8bpc_rvv:    351.1 ( 9.89x)
blend_v_w32_8bpc_c:     6832.1 ( 1.00x)
blend_v_w32_8bpc_rvv:    635.4 (10.75x)

SpacemiT K1

blend_v_w2_8bpc_c:       218.0 ( 1.00x)
blend_v_w2_8bpc_rvv:     144.3 ( 1.51x)
blend_v_w4_8bpc_c:       921.7 ( 1.00x)
blend_v_w4_8bpc_rvv:     237.1 ( 3.89x)
blend_v_w8_8bpc_c:      1739.8 ( 1.00x)
blend_v_w8_8bpc_rvv:     237.4 ( 7.33x)
blend_v_w16_8bpc_c:     3376.6 ( 1.00x)
blend_v_w16_8bpc_rvv:    296.3 (11.40x)
blend_v_w32_8bpc_c:     6647.2 ( 1.00x)
blend_v_w32_8bpc_rvv:    408.1 (16.29x)