Extend modref to track kills

Extend modref to track kills

This patch adds kill tracking to ipa-modref.  This is representd by array
of accesses to memory locations that are known to be overwritten by the
function.

gcc/ChangeLog:

2021-11-14  Jan Hubicka  <hubicka@ucw.cz>

	* ipa-modref-tree.c (modref_access_node::update_for_kills): New
	member function.
	(modref_access_node::merge_for_kills): Likewise.
	(modref_access_node::insert_kill): Likewise.
	* ipa-modref-tree.h (modref_access_node::update_for_kills,
	modref_access_node::merge_for_kills, modref_access_node::insert_kill):
	Declare.
	(modref_access_node::useful_for_kill): New member function.
	* ipa-modref.c (modref_summary::useful_p): Release useless kills.
	(lto_modref_summary): Add kills.
	(modref_summary::dump): Dump kills.
	(record_access): Add mdoref_access_node parameter.
	(record_access_lto): Likewise.
	(merge_call_side_effects): Merge kills.
	(analyze_call): Add ALWAYS_EXECUTED param and pass it around.
	(struct summary_ptrs): Add always_executed filed.
	(analyze_load): Update.
	(analyze_store): Update; record kills.
	(analyze_stmt): Add always_executed; record kills in clobbers.
	(analyze_function): Track always_executed.
	(modref_summaries::duplicate): Duplicate kills.
	(update_signature): Release kills.
	* ipa-modref.h (struct modref_summary): Add kills.
	* tree-ssa-alias.c (alias_stats): Add kill stats.
	(dump_alias_stats): Dump kill stats.
	(store_kills_ref_p): Break out from ...
	(stmt_kills_ref_p): Use it; handle modref info based kills.

gcc/testsuite/ChangeLog:

2021-11-14  Jan Hubicka  <hubicka@ucw.cz>

	* gcc.dg/tree-ssa/modref-dse-3.c: New test.