Improve handling of modref params.

Improve handling of modref params.

this patch makes insertion to modref access tree smarter when --param
modref-max-bases and moredref-max-refs are hit.  Instead of giving up
we either give up on base alias set (make it equal to ref) or turn the
alias set to 0.  This lets us to track useful info on quite large
functions, such as ggc_free.

gcc/ChangeLog:

	* ipa-modref-tree.c (test_insert_search_collapse): Update test.
	* ipa-modref-tree.h (modref_base_node::insert): Be smarter when
	hiting --param modref-max-refs limit.
	(modref_tree:insert_base): Be smarter when hitting
	--param modref-max-bases limit. Add new parameter REF.
	(modref_tree:insert): Update.
	(modref_tree:merge): Update.
	* ipa-modref.c (read_modref_records): Update.