lavc/pcm_tablegen: slight speedup of table generation

This gets rid of some branches to speed up table generation slightly
(impact higher on mulaw than alaw). Tables are identical to before,
tested with FATE.

Sample benchmark (Haswell, GNU/Linux+gcc):
old:
 313494 decicycles in build_alaw_table,    4094 runs,      2 skips
 315959 decicycles in build_alaw_table,    8190 runs,      2 skips

 323599 decicycles in build_ulaw_table,    4095 runs,      1 skips
 318849 decicycles in build_ulaw_table,    8188 runs,      4 skips

new:
 261902 decicycles in build_alaw_table,    4096 runs,      0 skips
 266519 decicycles in build_alaw_table,    8192 runs,      0 skips

 209657 decicycles in build_ulaw_table,    4096 runs,      0 skips
 232656 decicycles in build_ulaw_table,    8192 runs,      0 skips

Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Ganesh Ajjanagadde <gajjanagadde@gmail.com>

libavcodec/pcm_tablegen.h

@@ -87,21 +87,21 @@ static av_cold void build_xlaw_table(uint8_t *linear_to_xlaw,
 {
     int i, j, v, v1, v2;
 
-    j = 0;
-    for(i=0;i<128;i++) {
-        if (i != 127) {
-            v1 = xlaw2linear(i ^ mask);
-            v2 = xlaw2linear((i + 1) ^ mask);
-            v = (v1 + v2 + 4) >> 3;
-        } else {
-            v = 8192;
-        }
-        for(;j<v;j++) {
+    j = 1;
+    linear_to_xlaw[8192] = mask;
+    for(i=0;i<127;i++) {
+        v1 = xlaw2linear(i ^ mask);
+        v2 = xlaw2linear((i + 1) ^ mask);
+        v = (v1 + v2 + 4) >> 3;
+        for(;j<v;j+=1) {
+            linear_to_xlaw[8192 - j] = (i ^ (mask ^ 0x80));
             linear_to_xlaw[8192 + j] = (i ^ mask);
-            if (j > 0)
-                linear_to_xlaw[8192 - j] = (i ^ (mask ^ 0x80));
         }
     }
+    for(;j<8192;j++) {
+        linear_to_xlaw[8192 - j] = (127 ^ (mask ^ 0x80));
+        linear_to_xlaw[8192 + j] = (127 ^ mask);
+    }
     linear_to_xlaw[0] = linear_to_xlaw[1];
 }