tree-optimization/98113 - vectorize a sequence of BIT_INSERT_EXPRs

tree-optimization/98113 - vectorize a sequence of BIT_INSERT_EXPRs

This adds the capability to handle a sequence of vector BIT_INSERT_EXPRs
to be vectorized similar as to how we vectorize vector constructors.

2020-12-03  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/98113
	* tree-vectorizer.h (struct slp_root): New.
	(_bb_vec_info::roots): New member.
	* tree-vect-slp.c (vect_analyze_slp): Also walk BB info
	roots.
	(_bb_vec_info::_bb_vec_info): Adjust.
	(_bb_vec_info::~_bb_vec_info): Likewise.
	(vld_cmp): New.
	(vect_slp_is_lane_insert): Likewise.
	(vect_slp_check_for_constructors): Match a series of
	BIT_INSERT_EXPRs as vector constructor.
	(vect_slp_analyze_bb_1): Continue if BB info roots is
	not empty.
	(vect_slp_analyze_bb_1): Mark the whole BIT_INSERT_EXPR root
	sequence as pure_slp.

	* gcc.dg/vect/bb-slp-70.c: New testcase.