Come up with -fipa-reference-addressable flag.

Come up with -fipa-reference-addressable flag.

2018-11-09  Martin Liska  <mliska@suse.cz>

	* cgraph.h (ipa_discover_readonly_nonaddressable_vars): Rename
	to ...
	(ipa_discover_variable_flags): ... this.
	* common.opt: Come up with new flag -fipa-reference-addressable.
	* doc/invoke.texi: Document it.
	* ipa-reference.c (propagate): Call the renamed fn.
	* ipa-visibility.c (whole_program_function_and_variable_visibility):
	Likewise.
	* ipa.c (ipa_discover_readonly_nonaddressable_vars): Renamed to
	...
	(ipa_discover_variable_flags): ... this.  Discover
	non-addressable variables only with the newly added flag.
	* opts.c: Enable the newly added flag with -O1 and higher
	optimization level.
2018-11-09  Martin Liska  <mliska@suse.cz>

	* gcc.dg/tree-ssa/writeonly-2.c: New test.

From-SVN: r265969