Introduce flag_cunroll_grow_size for cunroll and avoid enable it at -O2

Introduce flag_cunroll_grow_size for cunroll and avoid enable it at -O2

Currently GIMPLE complete unroller(cunroll) is checking
flag_unroll_loops and flag_peel_loops to see if allow size growth.
Beside affects curnoll, flag_unroll_loops also controls RTL unroler.
To have more freedom to control cunroll and RTL unroller, this patch
introduces flag_cunroll_grow_size.  With this patch, we can control
cunroll and RTL unroller indepently. And enable flag_cunroll_grow_size
only if -funroll-loops or -fpeel-loops or -O3 is specified explicitly.

gcc/ChangeLog
2020-06-19  Jiufu Guo  <guojiufu@linux.ibm.com>

	PR target/95018
	* common.opt (flag_cunroll_grow_size): New flag.
	* toplev.c (process_options): Set flag_cunroll_grow_size.
	* tree-ssa-loop-ivcanon.c (pass_complete_unroll::execute):
	Use flag_cunroll_grow_size.
	* config/rs6000/rs6000.c (rs6000_option_override_internal):
	Override flag_cunroll_grow_size.