riscv64/mc: Branchless vsetvl in blend function

Kendryte K230

blend_w4_8bpc_c:       204.8 ( 1.00x)
blend_w4_8bpc_rvv:      59.8 ( 3.42x)
blend_w8_8bpc_c:       608.9 ( 1.00x)
blend_w8_8bpc_rvv:      87.2 ( 6.98x)
blend_w16_8bpc_c:     2362.4 ( 1.00x)
blend_w16_8bpc_rvv:    225.2 (10.49x)
blend_w32_8bpc_c:     5990.4 ( 1.00x)
blend_w32_8bpc_rvv:    518.3 (11.56x)

SpacemiT K1

blend_w4_8bpc_c:       201.6 ( 1.00x)
blend_w4_8bpc_rvv:      58.0 ( 3.48x)
blend_w8_8bpc_c:       595.1 ( 1.00x)
blend_w8_8bpc_rvv:      82.1 ( 7.25x)
blend_w16_8bpc_c:     2308.8 ( 1.00x)
blend_w16_8bpc_rvv:    189.0 (12.22x)
blend_w32_8bpc_c:     5853.1 ( 1.00x)
blend_w32_8bpc_rvv:    339.5 (17.24x)