rs6000: Refine RTL unroll hook for small loops

rs6000: Refine RTL unroll hook for small loops

For very small loops (< 6 insns), it would be fine to unroll 4
times to run fast with less latency and better cache usage.  Like
below loops:
 while (i) a[--i] = NULL;   while (p < e)  *d++ = *p++;

With this patch enhances, we could see some performance improvement
for some workloads(e.g. SPEC2017).

2020-07-13  Jiufu Guo   <guojiufu@cn.ibm.com>

	* config/rs6000/rs6000.c (rs6000_loop_unroll_adjust): Refine hook.