Use modref summaries for byte-wise dead store elimination.

Use modref summaries for byte-wise dead store elimination.

gcc/ChangeLog:

	* ipa-modref.c (get_modref_function_summary): Declare.
	* ipa-modref.h (get_modref_function_summary): New function.
	* tree-ssa-dse.c (clear_live_bytes_for_ref): Break out from ...
	(clear_bytes_written_by): ... here; also clear memory killed by
	calls.

gcc/testsuite/ChangeLog:

	* gcc.dg/tree-ssa/modref-dse-4.c: New test.