Disable some patterns for fold-left reductions (PR 83965)

Disable some patterns for fold-left reductions (PR 83965)

In this PR we recognised a PLUS_EXPR as a fold-left reduction,
then applied pattern matching to convert it to a WIDEN_SUM_EXPR.
We need to keep the original code in this case since we implement
the reduction using scalar rather than vector operations.

2018-01-23  Richard Sandiford  <richard.sandiford@linaro.org>

gcc/
	PR tree-optimization/83965
	* tree-vect-patterns.c (vect_reassociating_reduction_p): New function.
	(vect_recog_dot_prod_pattern, vect_recog_sad_pattern): Use it
	instead of checking only for a reduction.
	(vect_recog_widen_sum_pattern): Likewise.

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

From-SVN: r256976