Fix vec_cmp comparison mode

vec_cmps assign the result of a vector comparison to a mask.
The optab was called with the destination having mode mask_mode
but with the source (the comparison) having mode VOIDmode,
which led to invalid rtl if the source operand was used directly.

gcc/
2016-11-15  Richard Sandiford  <richard.sandiford@arm.com>
	    Alan Hayward  <alan.hayward@arm.com>
	    David Sherwood  <david.sherwood@arm.com>

	* optabs.c (vector_compare_rtx): Add a cmp_mode parameter
	and use it in the final call to gen_rtx_fmt_ee.
	(expand_vec_cond_expr): Update accordingly.
	(expand_vec_cmp_expr): Likewise.

Co-Authored-By: Alan Hayward <alan.hayward@arm.com>
Co-Authored-By: David Sherwood <david.sherwood@arm.com>

From-SVN: r242489