With the new reduction vectype handling, neutral_op_for_slp_reduction
needs to know whether the caller is using STMT_VINFO_REDUC_VECTYPE
(for an epilogue value) or STMT_VINFO_VECTYPE (for a PHI argument).

This fixes various gcc.target/aarch64/sve/slp_* tests.

2019-11-08  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* tree-vect-loop.c (neutral_op_for_slp_reduction): Take the
	vector type as an argument rather than reading it from the
	stmt_vec_info.
	(vect_create_epilog_for_reduction): Update accordingly.
	(vectorizable_reduction): Likewise.
	(vect_transform_cycle_phi): Likewise.

From-SVN: r277977