rs6000: Refine small loop unroll in loop_unroll_adjust hook

rs6000: Refine small loop unroll in loop_unroll_adjust hook

In this patch, loop unroll adjust hook is introduced for powerpc.  We
can do target related heuristic adjustment in this hook.  In this patch,
-funroll-loops is enabled for small loops at O2 and above with an option
-munroll-small-loops to guard the small loops unrolling, and it works
fine with -flto.

gcc/
2019-11-11  Jiufu Guo  <guojiufu@linux.ibm.com>

	PR tree-optimization/88760
	* gcc/config/rs6000/rs6000.opt (-munroll-only-small-loops): New option.
	* gcc/common/config/rs6000/rs6000-common.c
	(rs6000_option_optimization_table) [OPT_LEVELS_2_PLUS_SPEED_ONLY]:
	Turn on -funroll-loops and -munroll-only-small-loops.
	[OPT_LEVELS_ALL]: Turn off -fweb and -frename-registers.
	* config/rs6000/rs6000.c (rs6000_option_override_internal): Remove
	set of PARAM_MAX_UNROLL_TIMES and PARAM_MAX_UNROLLED_INSNS.
	Turn off -munroll-only-small-loops for explicit -funroll-loops.
	(TARGET_LOOP_UNROLL_ADJUST): Add loop unroll adjust hook.
	(rs6000_loop_unroll_adjust): Define it.  Use -munroll-only-small-loops.

gcc.testsuite/
2019-11-11  Jiufu Guo  <guojiufu@linux.ibm.com>

	PR tree-optimization/88760
	* gcc.dg/pr59643.c: Update back to r277550.

From-SVN: r278034