re PR rtl-optimization/11059 (empty union optimization ice)

	PR optimization/11059
	* expr.c (can_store_by_pieces): Return true if length is zero.
	(store_by_pieces): If length is zero and endp is two, abort,
	othwerise, if length is zero and endp is not two, return "to".
	(clear_by_pieces): Do nothing if length is zero.
	(clear_storage): Do nothing if length is zero.
	(store_constructor): Simplify code when size is zero, or the
	target has already been cleared.  This avoids emitting a
	blockage instruction when initializing empty structures.

	* g++.dg/opt/emptyunion.C: New testcase.

From-SVN: r69049