We weren't checking whether the phi in a conditional reduction was
used by the condition itself (which isn't a case we handle).

2017-08-11  Richard Sandiford  <richard.sandiford@linaro.org>

gcc/
	PR tree-optimization/81835
	* tree-vect-loop.c (vect_is_simple_reduction): Simply checks for
	the phi SSA_NAME.  Check that the condition in a COND_EXPR does
	not depend on the phi.

gcc/testsuite/
	PR tree-optimization/81835
	* gcc.dg/vect/pr81815.c: New test.

From-SVN: r251117