This patch prevents pattern-matching of fold-left SLP reduction chains,
which the previous patch for 83965 didn't handle properly.  It only
stops the last statement in the group from being matched, but that's
enough to cause the group to be dissolved later.

A better fix would be to put all the information about the reduction
on the the first statement in the reduction chain, so that every
statement in the group can tell what the group is doing.  That doesn't
seem like stage 4 material though.

2018-02-26  Richard Sandiford  <richard.sandiford@linaro.org>

gcc/
	PR tree-optimization/83965
	* tree-vect-patterns.c (vect_reassociating_reduction_p): Assume
	that grouped statements are part of a reduction chain.  Return
	true if the statement is not marked as a reduction itself but
	is part of a group.
	(vect_recog_dot_prod_pattern): Don't check whether the statement
	is part of a group here.
	(vect_recog_sad_pattern): Likewise.
	(vect_recog_widen_sum_pattern): Likewise.

gcc/testsuite/
	PR tree-optimization/83965
	* gcc.dg/vect/pr83965-2.c: New test.

From-SVN: r257995