[25/46] Make get_earlier/later_stmt take and return stmt_vec_infos

[25/46] Make get_earlier/later_stmt take and return stmt_vec_infos

...and also make vect_find_last_scalar_stmt_in_slp return a stmt_vec_info.

2018-07-31  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* tree-vectorizer.h (get_earlier_stmt, get_later_stmt): Take and
	return stmt_vec_infos rather than gimple stmts.  Do not accept
	null arguments.
	(vect_find_last_scalar_stmt_in_slp): Return a stmt_vec_info instead
	of a gimple stmt.
	* tree-vect-slp.c (vect_find_last_scalar_stmt_in_slp): Likewise.
	Update use of get_later_stmt.
	(vect_get_constant_vectors): Update call accordingly.
	(vect_schedule_slp_instance): Likewise
	* tree-vect-data-refs.c (vect_slp_analyze_node_dependences): Likewise.
	(vect_slp_analyze_instance_dependence): Likewise.
	(vect_preserves_scalar_order_p): Update use of get_earlier_stmt.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@263140 138bc75d-0d04-0410-961f-82ee72b054a4