lavc/lpc: optimise RVV vector type for compute_autocorr

On SpacemiT X60 (with len == 4000):
autocorr_10_c:       2303.7
autocorr_10_rvv_f64: 1411.5 (before)
autocorr_10_rvv_f64:  842.2 (after)

libavcodec/riscv/lpc_init.c

@@ -36,7 +36,8 @@ av_cold void ff_lpc_init_riscv(LPCContext *c)
     if ((flags & AV_CPU_FLAG_RVV_F64) && (flags & AV_CPU_FLAG_RVB_ADDR)) {
         c->lpc_apply_welch_window = ff_lpc_apply_welch_window_rvv;
 
-        if (ff_get_rv_vlenb() > c->max_order)
+        if ((flags & AV_CPU_FLAG_RVB_BASIC) &&
+            ff_get_rv_vlenb() > c->max_order)
             c->lpc_compute_autocorr = ff_lpc_compute_autocorr_rvv;
     }
 #endif

libavcodec/riscv/lpc_rvv.S

@@ -86,10 +86,11 @@ func ff_lpc_apply_welch_window_rvv, zve64d
         ret
 endfunc
 
-func ff_lpc_compute_autocorr_rvv, zve64d
+func ff_lpc_compute_autocorr_rvv, zve64d, zbb
         addi      a2, a2, 1
+        vtype_vli t1, a2, t2, e64, ta, ma
         li        t0, 1
-        vsetvli   zero, a2, e64, m8, ta, ma
+        vsetvl    zero, a2, t1
         fcvt.d.l  ft0, t0
         vle64.v   v0, (a0)
         sh3add    a0, a2, a0   # data += lag