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>