Move SLP nodes to an alloc-pool

Move SLP nodes to an alloc-pool

This introduces a global alloc-pool for SLP nodes to reduce overhead
on SLP allocation churn which will get worse and to eventually release
SLP cycles which will retain a refcount of one and thus are never
freed at the moment.

2020-10-26  Richard Biener  <rguenther@suse.de>

	* tree-vectorizer.h (slp_tree_pool): Declare.
	(_slp_tree::operator new): Likewise.
	(_slp_tree::operator delete): Likewise.
	* tree-vectorizer.c (vectorize_loops): Allocate and free the
	slp_tree_pool.
	(pass_slp_vectorize::execute): Likewise.
	* tree-vect-slp.c (slp_tree_pool): Define.
	(_slp_tree::operator new): Likewise.
	(_slp_tree::operator delete): Likewise.