tree-optimization/97630 - fix SLP cycle memory leak

tree-optimization/97630 - fix SLP cycle memory leak

This fixes SLP cycles leaking memory by maintaining a double-linked
list of allocatd SLP nodes we can zap when we free the alloc pool.

2020-12-02  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/97630
	* tree-vectorizer.h (_slp_tree::next_node,
	_slp_tree::prev_node): New.
	(vect_slp_init): Declare.
	(vect_slp_fini): Likewise.
	* tree-vectorizer.c (vectorize_loops): Call vect_slp_init/fini.
	(pass_slp_vectorize::execute): Likewise.
	* tree-vect-slp.c (vect_slp_init): New.
	(vect_slp_fini): Likewise.
	(slp_first_node): New global.
	(_slp_tree::_slp_tree): Link node into the SLP tree list.
	(_slp_tree::~_slp_tree): Delink node from the SLP tree list.