riscv64/mc: Branchless vsetvl in blend_h function

Kendryte K230

blend_h_w2_8bpc_c:        165.9 ( 1.00x)
blend_h_w2_8bpc_rvv:       83.8 ( 1.98x)
blend_h_w4_8bpc_c:        295.2 ( 1.00x)
blend_h_w4_8bpc_rvv:       83.8 ( 3.52x)
blend_h_w8_8bpc_c:        557.9 ( 1.00x)
blend_h_w8_8bpc_rvv:       92.5 ( 6.03x)
blend_h_w16_8bpc_c:      1078.8 ( 1.00x)
blend_h_w16_8bpc_rvv:     117.3 ( 9.19x)
blend_h_w32_8bpc_c:      2117.8 ( 1.00x)
blend_h_w32_8bpc_rvv:     200.5 (10.57x)
blend_h_w64_8bpc_c:      4194.7 ( 1.00x)
blend_h_w64_8bpc_rvv:     363.2 (11.55x)
blend_h_w128_8bpc_c:    10271.4 ( 1.00x)
blend_h_w128_8bpc_rvv:    844.5 (12.16x)

SpacemiT K1

blend_h_w2_8bpc_c:        162.5 ( 1.00x)
blend_h_w2_8bpc_rvv:       83.9 ( 1.94x)
blend_h_w4_8bpc_c:        288.6 ( 1.00x)
blend_h_w4_8bpc_rvv:       83.7 ( 3.45x)
blend_h_w8_8bpc_c:        544.7 ( 1.00x)
blend_h_w8_8bpc_rvv:       84.0 ( 6.48x)
blend_h_w16_8bpc_c:      1052.8 ( 1.00x)
blend_h_w16_8bpc_rvv:     102.9 (10.23x)
blend_h_w32_8bpc_c:      2068.0 ( 1.00x)
blend_h_w32_8bpc_rvv:     131.4 (15.73x)
blend_h_w64_8bpc_c:      4093.7 ( 1.00x)
blend_h_w64_8bpc_rvv:     220.3 (18.58x)
blend_h_w128_8bpc_c:    10023.1 ( 1.00x)
blend_h_w128_8bpc_rvv:    467.3 (21.45x)