Use get_related_vectype_for_scalar_type for reduction indices

The related_vector_mode series missed this case in
vect_create_epilog_for_reduction, where we want to create the
unsigned integer equivalent of another vector.  Without it we
could mix SVE and Advanced SIMD vectors in the same operation.

This showed up on existing tests when testing with fixed-length
-msve-vector-bits=128.

2020-01-10  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* tree-vect-loop.c (vect_create_epilog_for_reduction): Use
	get_related_vectype_for_scalar_type rather than build_vector_type
	to create the index type for a conditional reduction.

From-SVN: r280112