Merge load/stores in ipa-modref summaries

Merge load/stores in ipa-modref summaries

this patch adds logic needed to merge neighbouring accesses in ipa-modref
summaries.  This helps analyzing array initializers and similar code.  It is
bit of work, since it breaks the fact that modref tree makes a good lattice for
dataflow: the access ranges can be extended indefinitely.  For this reason I
added counter tracking number of adjustments and a cap to limit them during the
dataflow.

gcc/ChangeLog:

	* doc/invoke.texi: Document --param modref-max-adjustments.
	* ipa-modref-tree.c (test_insert_search_collapse): Update.
	(test_merge): Update.
	* ipa-modref-tree.h (struct modref_access_node): Add adjustments;
	(modref_access_node::operator==): Fix handling of access ranges.
	(modref_access_node::contains): Constify parameter; handle also
	mismatched parm offsets.
	(modref_access_node::update): New function.
	(modref_access_node::merge): New function.
	(unspecified_modref_access_node): Update constructor.
	(modref_ref_node::insert_access): Add record_adjustments parameter;
	handle merging.
	(modref_ref_node::try_merge_with): New private function.
	(modref_tree::insert): New record_adjustments parameter.
	(modref_tree::merge): New record_adjustments parameter.
	(modref_tree::copy_from): Update.
	* ipa-modref.c (dump_access): Dump adjustments field.
	(get_access): Update constructor.
	(record_access): Update call of insert.
	(record_access_lto): Update call of insert.
	(merge_call_side_effects): Add record_adjustments parameter.
	(get_access_for_fnspec): Update.
	(process_fnspec): Update.
	(analyze_call): Update.
	(analyze_function): Update.
	(read_modref_records): Update.
	(ipa_merge_modref_summary_after_inlining): Update.
	(propagate_unknown_call): Update.
	(modref_propagate_in_scc): Update.
	* params.opt (param-max-modref-adjustments=): New.

gcc/testsuite/ChangeLog:

	* gcc.dg/ipa/modref-1.c: Update testcase.
	* gcc.dg/tree-ssa/modref-4.c: Update testcase.
	* gcc.dg/tree-ssa/modref-8.c: New test.