[10/n] PR85694: Split out check for vectorizable associative reductions

[10/n] PR85694: Split out check for vectorizable associative reductions

This patch adds an overload of vect_reassociating_reduction_p
that checks for a vectorizable associative reduction,
since the check was duplicated in three functions.

2018-06-30  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* tree-vect-patterns.c (vect_reassociating_reduction_p): New function.
	(vect_recog_dot_prod_pattern, vect_recog_sad_pattern)
	(vect_recog_widen_sum_pattern): Use it.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@262274 138bc75d-0d04-0410-961f-82ee72b054a4