[patch] Prefetch: adjust prefetch_mod to unroll_factor ratio threshold

[Fang, Changpeng]

Hi, 

Attached is the patch to adjust the prefetch_mod to unroll_factor ratio threshold
from 8 to 4. In certain sense, this adjustment means that we do not generate a prefetch
if it introduces 7 or more useless prefetches. 

This patch improves 416.gamess by 2~3% on amd-linux64 systems.

patch passed bootstrapping and gcc regression tests.

Is it OK for the trunk?

Thanks,

Changpeng  
 
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