lavc/flacdsp: optimise RVV vector type for lpc16

This calculates the optimal vector type value at run-time based on the
hardware vector length and the FLAC LPC prediction order. In this
particular case, the additional computation is easily amortised over
the loop iterations:

T-Head C908:
    C     V before  V after
 1   48.0    214.7     95.2
 2   64.7    214.2     94.7
 3   79.7    213.5     94.5
 4   96.2    196.5     94.2 #
 5  111.0    195.7    118.5
 6  127.0    211.2    102.0
 7  143.7    194.2    101.5
 8  175.7    193.2    101.2 #
 9  176.2    224.2    126.0
10  191.5    192.0    125.5
11  224.5    191.2    124.7
12  223.0    190.2    124.2
13  239.2    189.5    123.7
14  253.7    188.7    139.5
15  286.2    188.0    122.7
16  284.0    187.0    122.5 #
17  300.2    186.5    186.5
18  314.0    185.5    185.7
19  329.7    184.7    185.0
20  343.0    184.2    184.2
21  358.7    199.2    183.7
22  371.7    182.7    182.7
23  387.5    181.7    182.0
24  400.7    181.0    181.2
25  431.5    180.2    196.5
26  443.7    195.5    196.0
27  459.0    178.7    196.2
28  470.7    177.7    194.2
29  470.0    177.0    193.5
30  481.2    176.2    176.5
31  496.2    175.5    175.7
32  507.2    174.7    191.0 #

 # Power of two boundary.

With 128-bit vectors, improvements are expected for the first two
test cases only. For the other two, there is overhead but below noise.
Improvements should be better observable with prediction order of 8
and less, or on hardware with larger vector sizes.

libavcodec/riscv/flacdsp_init.c

@@ -71,7 +71,7 @@ av_cold void ff_flacdsp_init_riscv(FLACDSPContext *c, enum AVSampleFormat fmt,
     if ((flags & AV_CPU_FLAG_RVV_I32) && (flags & AV_CPU_FLAG_RVB_ADDR)) {
         int vlenb = ff_get_rv_vlenb();
 
-        if (vlenb >= 16)
+        if ((flags & AV_CPU_FLAG_RVB_BASIC) && vlenb >= 16)
             c->lpc16 = ff_flac_lpc16_rvv;
 
 # if (__riscv_xlen >= 64)

libavcodec/riscv/flacdsp_rvv.S

@@ -20,8 +20,9 @@
 
 #include "libavutil/riscv/asm.S"
 
-func ff_flac_lpc16_rvv, zve32x
-        vsetvli zero, a2, e32, m8, ta, ma
+func ff_flac_lpc16_rvv, zve32x, zbb
+        vtype_vli t0, a2, t2, e32, ta, ma
+        vsetvl  zero, a2, t0
         vle32.v v8, (a1)
         sub     a4, a4, a2
         vle32.v v16, (a0)