tree-optimization/121395 - SLP of SIMD calls w/o LHS

tree-optimization/121395 - SLP of SIMD calls w/o LHS

The following records the alternate SLP instance entries coming from
stmts with stores that have no SSA def, like OMP SIMD calls without LHS.
There's a bit of fallout with having a SLP tree with a NULL vectype,
but nothing too gross.

	PR tree-optimization/121395
	* tree-vectorizer.h (_loop_vec_info::alternate_defs): New member.
	(LOOP_VINFO_ALTERNATE_DEFS): New.
	* tree-vect-stmts.cc (vect_stmt_relevant_p): Populate it.
	(vectorizable_simd_clone_call): Do not register a SLP def
	when there is none.
	* tree-vect-slp.cc (vect_build_slp_tree_1): Allow a NULL
	vectype when there's no LHS.  Allow all calls w/o LHS.
	(vect_analyze_slp): Process LOOP_VINFO_ALTERNATE_DEFS as
	SLP graph entries.
	(vect_make_slp_decision): Handle a NULL SLP_TREE_VECTYPE.
	(vect_slp_analyze_node_operations_1): Likewise.
	(vect_schedule_slp_node): Likewise.

	* gcc.dg/vect/pr59984.c: Adjust.