middle-end: Refactor refcnt to use SLP_TREE_REF_COUNT for consistency

middle-end: Refactor refcnt to use SLP_TREE_REF_COUNT for consistency

This is a small refactoring which introduces SLP_TREE_REF_COUNT and replaces
the uses of refcnt with it.  This for consistency between the other properties.

A similar patch was pre-approved last year but since there are more use now I am
sending it for review anyway.

gcc/ChangeLog:

	* tree-vectorizer.h (SLP_TREE_REF_COUNT): New.
	* tree-vect-slp.c (_slp_tree::_slp_tree, _slp_tree::~_slp_tree,
	vect_free_slp_tree, vect_build_slp_tree, vect_print_slp_tree,
	slp_copy_subtree, vect_attempt_slp_rearrange_stmts): Use it.