rs6000: re-enable web and rnreg with -funroll-loops

Previously, limited unrolling was enabled at O2 for powerpc in r278034.  At that
time, -fweb and -frename-registers were not enabled together with -funroll-loops
even for -O3.  After that, we notice there are some performance degradations on
SPEC2006fp which caused by without web and rnreg.  This patch enable -fweb
and -frename-registers for -funroll-loops to align original behavior before
r278034.

gcc/
2019-12-23  Jiufu Guo  <guojiufu@linux.ibm.com>

	* config/rs6000/rs6000.c (rs6000_option_override_internal): Enable
	-fweb and -frename-registers with -funroll-loops

From-SVN: r279725