middle-end/96466 - fix VEC_COND isel/expansion issue

middle-end/96466 - fix VEC_COND isel/expansion issue

We need to avoid forcing BLKmode for truth vectors, instead do as
other code and use VOIDmode so layout_type can pick a suitable and
consistent mode.  RTL expansion of vect_cond_mask also needs to deal
with CONST_INT operands which means passing the mode explicitely.

2020-09-23  Richard Biener  <rguenther@suse.de>

	PR middle-end/96466
	* internal-fn.c (expand_vect_cond_mask_optab_fn): Use
	appropriate mode for force_reg.
	* tree.c (build_truth_vector_type_for): Pass VOIDmode to
	make_vector_type.

	* gcc.dg/pr96466.c: New testcase.