avoid stmt-info allocation for debug stmts

avoid stmt-info allocation for debug stmts

The following avoids allocating stmt info structs for debug stmts.

2020-06-10  Richard Biener  <rguenther@suse.de>

	* tree-vect-loop.c (vect_determine_vectorization_factor):
	Skip debug stmts.
	(_loop_vec_info::_loop_vec_info): Likewise.
	(vect_update_vf_for_slp): Likewise.
	(vect_analyze_loop_operations): Likewise.
	(update_epilogue_loop_vinfo): Likewise.
	* tree-vect-patterns.c (vect_determine_precisions): Likewise.
	(vect_pattern_recog): Likewise.
	* tree-vect-slp.c (vect_detect_hybrid_slp): Likewise.
	(_bb_vec_info::_bb_vec_info): Likewise.
	* tree-vect-stmts.c (vect_mark_stmts_to_be_vectorized):
	Likewise.