lavc/vc1dsp: fix overflow in R-V V inv_trans_8

The last set of additions/subtractions can break the 16-bit limit, and
require 17 bits of precision. This uses widening adds accordingly to fix
the MSS2 FATE tests.

The problem potentially also affects inv_trans_4 with a very low
probability, but this is not reproducible under FATE.

libavcodec/riscv/vc1dsp_rvv.S

@@ -141,44 +141,49 @@ func ff_vc1_inv_trans_8_rvv, zve32x
         vadd.vv v20, v20, v21
         vadd.vv v22, v22, v23
         vsll.vi v21, v3, 2
-        vadd.vv v16, v20, v22 # t1
+        vadd.vv v24, v20, v22 # t1
         vmul.vx v20, v1, t5
         vsll.vi v22, v5, 4
         vmul.vx v23, v7, t3
         vsub.vv v20, v20, v21
         vadd.vv v22, v22, v23
         vsll.vi v21, v3, 4
-        vsub.vv v17, v20, v22 # t2
+        vsub.vv v25, v20, v22 # t2
         vmul.vx v20, v1, t3
         vsll.vi v22, v5, 2
         vmul.vx v23, v7, t5
         vsub.vv v20, v20, v21
         vadd.vv v22, v22, v23
         vmul.vx v21, v3, t3
-        vadd.vv v18, v20, v22 # t3
+        vadd.vv v26, v20, v22 # t3
         vsll.vi v20, v1, 2
         vmul.vx v22, v5, t5
         vsll.vi v23, v7, 4
         vsub.vv v20, v20, v21
         vsub.vv v22, v22, v23
+        vadd.vv v27, v20, v22 # t4
         srli    t2, t1, 2
-        vadd.vv v0, v28, v16
-        vadd.vv v19, v20, v22 # t4
-        vadd.vv v1, v29, v17
-        vadd.vv v2, v30, v18
-        vadd.vv v3, v31, v19
-        vsub.vv v4, v31, v19
-        vsub.vv v5, v30, v18
-        vsub.vv v6, v29, v17
-        vsub.vv v7, v28, v16
+        vwadd.vv  v8, v28, v24
+        vwadd.vv  v10, v29, v25
+        vwadd.vv  v12, v30, v26
+        vwadd.vv  v14, v31, v27
         beqz    t2, 1f # faster than 4x add t2=zero
-        .irp    n,4,5,6,7
+        .irp    n,31,30,29,28
         vadd.vi v\n, v\n, 1
         .endr
 1:
-        .irp n,0,1,2,3,4,5,6,7
-        vssra.vx v\n, v\n, t1
-        .endr
+        vwsub.vv  v16, v31, v27
+        vwsub.vv  v18, v30, v26
+        vwsub.vv  v20, v29, v25
+        vwsub.vv  v22, v28, v24
+        vnclip.wx v0, v8, t1
+        vnclip.wx v1, v10, t1
+        vnclip.wx v2, v12, t1
+        vnclip.wx v3, v14, t1
+        vnclip.wx v4, v16, t1
+        vnclip.wx v5, v18, t1
+        vnclip.wx v6, v20, t1
+        vnclip.wx v7, v22, t1
         jr      t0
 endfunc