tree-optimization/97832 - handle associatable chains in SLP discovery

tree-optimization/97832 - handle associatable chains in SLP discovery

This makes SLP discovery handle associatable (including mixed
plus/minus) chains better by swapping operands across the whole
chain.  To work this adds caching of the 'matches' lanes for
failed SLP discovery attempts, thereby fixing a failed SLP
discovery for the slp-pr98855.cc testcase which results in
building an operand from scalars as expected.  Unfortunately
this makes us trip over the cost threshold so I'm XFAILing the
testcase for now.

For BB vectorization all this doesn't work because we have no way
to distinguish good from bad associations as we eventually build
operands from scalars and thus not fail in the classical sense.

2021-05-31  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/97832
	* tree-vectorizer.h (_slp_tree::failed): New.
	* tree-vect-slp.c (_slp_tree::_slp_tree): Initialize
	failed member.
	(_slp_tree::~_slp_tree): Free failed.
	(vect_build_slp_tree): Retain failed nodes and record
	matches in them, copying that back out when running
	into a cached fail.  Dump start and end of discovery.
	(dt_sort_cmp): New.
	(vect_build_slp_tree_2): Handle associatable chains
	together doing more aggressive operand swapping.

	* gcc.dg/vect/pr97832-1.c: New testcase.
	* gcc.dg/vect/pr97832-2.c: Likewise.
	* gcc.dg/vect/pr97832-3.c: Likewise.
	* g++.dg/vect/slp-pr98855.cc: XFAIL.