middle-end: Refactor and expose some vectorizer helper functions.

middle-end: Refactor and expose some vectorizer helper functions.

This is a small refactoring which exposes some helper functions in the
vectorizer so they can be used in other places.

gcc/ChangeLog:

	* tree-vect-patterns.c (vect_mark_pattern_stmts): Remove static inline.
	* tree-vect-slp.c (vect_create_new_slp_node): Remove static and only
	set smts if valid.
	* tree-vectorizer.c (vec_info::add_pattern_stmt): New.
	(vec_info::set_vinfo_for_stmt): Optionally enforce read-only.
	* tree-vectorizer.h (struct _slp_tree): Use new types.
	(lane_permutation_t, lane_permutation_t): New.
	(vect_create_new_slp_node, vect_mark_pattern_stmts): New.