add ctor/dtor to slp_tree

This adds constructor and destructor to slp_tree factoring common
code.  I've not changed the wrappers to overloaded CTORs since
I hope to use object_allocator<> and am not sure whether that can
be done in any fancy way yet.

2020-05-22  Richard Biener  <rguenther@suse.de>

	* tree-vectorizer.h (_slp_tree::_slp_tree): New.
	(_slp_tree::~_slp_tree): Likewise.
	* tree-vect-slp.c (_slp_tree::_slp_tree): Factor out code
	from allocators.
	(_slp_tree::~_slp_tree): Implement.
	(vect_free_slp_tree): Simplify.
	(vect_create_new_slp_node): Likewise.  Add nops parameter.
	(vect_build_slp_tree_2): Adjust.
	(vect_analyze_slp_instance): Likewise.