Fix reduc_index==1 handling for COND_REDUCTION (PR91909)

The then/else order of the VEC_COND_EXPRs created by
vect_create_epilog_for_reduction meeds to line up with the
main VEC_COND_EXPR.

2019-09-27  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	PR tree-optimization/91909
	* tree-vect-loop.c (vect_create_epilog_for_reduction): Take a
	reduc_index parameter.  When handling COND_REDUCTION, make sure
	that the reduction phi operand is in the correct arm of the
	VEC_COND_EXPR.
	(vectorizable_reduction): Pass reduc_index to the above.

From-SVN: r276175