Improve handling of table overflows in modref_ref_node

Improve handling of table overflows in modref_ref_node

gcc/ChangeLog:

	* ipa-modref-tree.h (modref_access_node::merge): Break out
	logic combining offsets and logic merging ranges to ...
	(modref_access_node::combined_offsets): ... here
	(modref_access_node::update2): ... here
	(modref_access_node::closer_pair_p): New member function.
	(modref_access_node::forced_merge): New member function.
	(modre_ref_node::insert): Do merging when table is full.

gcc/testsuite/ChangeLog:

	* gcc.dg/tree-ssa/modref-9.c: New test.