optimize permutes in SLP, remove vect_attempt_slp_rearrange_stmts

optimize permutes in SLP, remove vect_attempt_slp_rearrange_stmts

This introduces a permute optimization phase for SLP which is
intended to cover the existing permute eliding for SLP reductions
plus handling commonizing the easy cases.

It currently uses graphds to compute a postorder on the reverse
SLP graph and it handles all cases vect_attempt_slp_rearrange_stmts
did (hopefully - I've adjusted most testcases that triggered it
a few days ago).  It restricts itself to move around bijective
permutations to simplify things for now, mainly around constant nodes.

As a prerequesite it makes the SLP graph cyclic (ugh).  It looks
like it would pay off to compute a PRE/POST order visit array
once and elide all the recursive SLP graph walks and their
visited hash-set.  At least for the time where we do not change
the SLP graph during such walk.

I do not like using graphds too much but at least I don't have to
re-implement yet another RPO walk, so maybe it isn't too bad.

It now computes permute placement during iteration and thus should
get cycles more obviously correct.

Richard.

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

	* tree-vect-data-refs.c (vect_slp_analyze_instance_dependence):
	Use SLP_TREE_REPRESENTATIVE.
	* tree-vectorizer.h (_slp_tree::vertex): New member used
	for graphds interfacing.
	* tree-vect-slp.c (vect_build_slp_tree_2): Allocate space
	for PHI SLP children.
	(vect_analyze_slp_backedges): New function filling in SLP
	node children for PHIs that correspond to backedge values.
	(vect_analyze_slp): Call vect_analyze_slp_backedges for the
	graph.
	(vect_slp_analyze_node_operations): Deal with a cyclic graph.
	(vect_schedule_slp_instance): Likewise.
	(vect_schedule_slp): Likewise.
	(slp_copy_subtree): Remove.
	(vect_slp_rearrange_stmts): Likewise.
	(vect_attempt_slp_rearrange_stmts): Likewise.
	(vect_slp_build_vertices): New functions.
	(vect_slp_permute): Likewise.
	(vect_slp_perms_eq): Likewise.
	(vect_optimize_slp): Remove special code to elide
	permutations with SLP reductions.  Implement generic
	permute optimization.

	* gcc.dg/vect/bb-slp-50.c: New testcase.
	* gcc.dg/vect/bb-slp-51.c: Likewise.