[patch, PR44297] prefetch improvements to fix 416 regression from non-constant step prefetching

[Fang, Changpeng]

Hi,

Attached is the patch to limit non-constant step prefetching only to the innermost loops.
This patch fixes the 5% degradation in 416.gamess caused by non-constant step prefetching.

patch passed bootstrapping and gcc regression tests.

Is it OK for the trunk?

Thanks,

Changpeng
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