[02/11] Remove vect_schedule_slp return value

[02/11] Remove vect_schedule_slp return value

Nothing now uses the vect_schedule_slp return value, so it's not worth
propagating the value through vect_schedule_slp_instance.

2018-08-01  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* tree-vectorizer.h (vect_schedule_slp): Return void.
	* tree-vect-slp.c (vect_schedule_slp_instance): Likewise.
	(vect_schedule_slp): Likewise.

From-SVN: r263215