move permutation validity check

This delays the SLP permutation check to vectorizable_load and optimizes
permutations only after all SLP instances have been generated and the
vectorization factor is determined.

2020-05-08  Richard Biener  <rguenther@suse.de>

	* tree-vectorizer.h (vec_info::slp_loads): New.
	(vect_optimize_slp): Declare.
	* tree-vect-slp.c (vect_attempt_slp_rearrange_stmts):  Do
	nothing when there are no loads.
	(vect_gather_slp_loads): Gather loads into a vector.
	(vect_supported_load_permutation_p): Remove.
	(vect_analyze_slp_instance): Do not verify permutation
	validity here.
	(vect_analyze_slp): Optimize permutations of reductions
	after all SLP instances have been gathered and gather
	all loads.
	(vect_optimize_slp): New function split out from
	vect_supported_load_permutation_p.  Elide some permutations.
	(vect_slp_analyze_bb_1): Call vect_optimize_slp.
	* tree-vect-loop.c (vect_analyze_loop_2): Likewise.
	* tree-vect-stmts.c (vectorizable_load): Check whether
	the load can be permuted.  When generating code assert we can.

	* gcc.dg/vect/bb-slp-pr68892.c: Adjust for not supported
	SLP permutations becoming builds from scalars.
	* gcc.dg/vect/bb-slp-pr78205.c: Likewise.
	* gcc.dg/vect/bb-slp-34.c: Likewise.