omp-simd-clone: Unbreak bootstrap

This patch fixes -Werror=sign-compare errors during stage2/stage3.

2022-08-30  Rainer Orth  <ro@CeBiTec.Uni-Bielefeld.DE>
	    Jakub Jelinek  <jakub@redhat.com>

	* omp-simd-clone.cc (simd_clone_adjust_return_type,
	simd_clone_adjust_argument_types): Use known_eq (veclen, 0U)
	instead of known_eq (veclen, 0) to avoid -Wsign-compare warnings.