Rewrite the loops such that gcc can vectorize them using saturated-sub
on x86-64 architecture. Speedup the performance by some 7% on my linux box
with corei7 archiecture.

  The original loop is legal to be vectorized; gcc 4.7.* and 4.8.*
somehow fail to catch this case. There are still have room to squeeze
from the vectorized code. However, since these loops now account for about
1.5% of execution time, it is not worthwhile to sequeeze the performance
via hand-writing assembly.

  The original loops are guarded with "#ifdef NOT_TWEAK_COMPILER". By
default, the modified version is picked up unless the code is compiled
explictly with -DNOT_TWEAK_COMPILER.