Fix some side cases of side effects discovery

Fix some side cases of side effects discovery

I wrote script comparing modref pure/const discovery with ipa-pure-const
and found mistakes on both ends.  This plugs the modref differences in handling
looping pure consts which were previously missed due to early exits on
ECF_CONST | ECF_PURE.  Those early exists are bit anoying and I think as
a cleanup I may just drop some of them as premature optimizations coming from
time modref was very simplistic on what it propagates.

gcc/ChangeLog:

2021-11-11  Jan Hubicka  <hubicka@ucw.cz>

	* ipa-modref.c (modref_summary::useful_p): Check also for side-effects
	with looping const/pure.
	(modref_summary_lto::useful_p): Likewise.
	(merge_call_side_effects): Merge side effects before early exit
	for pure/const.
	(process_fnspec): Also handle pure functions.
	(analyze_call): Do not early exit on looping pure const.
	(propagate_unknown_call): Also handle nontrivial SCC as side-effect.
	(modref_propagate_in_scc): Update.