rs6000: allow cunroll to grow size according to -funroll-loop or -fpeel-loops

rs6000: allow cunroll to grow size according to -funroll-loop or -fpeel-loops

Previously, flag_unroll_loops was turned on at -O2 implicitly.  This
also turned on cunroll with allowance size increasing, and cunroll
will unroll/peel the loop even the loop is complex like code in PR95018.
With this patch, size growth for cunroll is allowed only for if -funroll-loops
or -fpeel-loops or -O3 is specified explicitly.

gcc/ChangeLog
2020-06-07  Jiufu Guo  <guojiufu@linux.ibm.com>

	PR target/95018
	* config/rs6000/rs6000.c (rs6000_option_override_internal):
	Override flag_cunroll_grow_size.