This patch changes LOOP_VINFO_REDUCTIONS from an auto_vec<gimple *>
to an auto_vec<stmt_vec_info>.  It also changes the associated
vect_force_simple_reduction so that it takes and returns stmt_vec_infos
instead of gimple stmts.

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

gcc/
	* tree-vectorizer.h (_loop_vec_info::reductions): Change from an
	auto_vec<gimple *> to an auto_vec<stmt_vec_info>.
	(vect_force_simple_reduction): Take and return stmt_vec_infos rather
	than gimple stmts.
	* tree-parloops.c (valid_reduction_p): Take a stmt_vec_info instead
	of a gimple stmt.
	(gather_scalar_reductions): Update after above interface changes.
	* tree-vect-loop.c (vect_analyze_scalar_cycles_1): Likewise.
	(vect_is_simple_reduction): Take and return stmt_vec_infos rather
	than gimple stmts.
	(vect_force_simple_reduction): Likewise.
	* tree-vect-patterns.c (vect_pattern_recog_1): Update use of
	LOOP_VINFO_REDUCTIONS.
	* tree-vect-slp.c (vect_analyze_slp_instance): Likewise.

From-SVN: r263132