[patch, PR44297] prefetch improvements to fix 465.tonto from non-constant step prefetching

[Fang, Changpeng]

Hi,

Attached is the patch to fix 465/tonto regression (> 9%) caused (or exposed) by non-constant step prefetching.
The regressions mainly comes from loops which have array references with both  constant (prefetch_mod = 8) 
and non-constant (prefetch_mod=1) steps. The loops are unrolled 8 times, and 8 non-constant step prefetches
 are inserted into the unrolled loops.

In this patch, we re-compute the prefetch count using prefetch_mod and unroll_factor after the unroll_factor 
is determined.

The patch passed bootstrapping and gcc regression tests on amd-linux64 system.

Is it OK for the trunk?

Thanks,

Changpeng