rs6000: enable -fweb for small loops unrolling

rs6000: enable -fweb for small loops unrolling

Previously -fweb was disabled if only unroll small loops.  After that
we find there is cases where it could help to rename pseudos and avoid
some anti-dependence which may occur after unroll.

This patch enables -fweb for small loops unrolling.

2020-04-27  Jiufu Guo   <guojiufu@cn.ibm.com>

	* common/config/rs6000/rs6000-common.c
	(rs6000_option_optimization_table) [OPT_LEVELS_ALL]: Remove turn off
	-fweb.
	* config/rs6000/rs6000.c (rs6000_option_override_internal): Avoid to
	set flag_web.