openacc: Turn off worker partitioning if num_workers==1

This patch turns off the middle-end worker-partitioning support if the
number of workers for an outlined offload function is one.  In that case,
we do not need to perform the broadcasting/neutering code transformation.

	gcc/
	* omp-oacc-neuter-broadcast.cc
	(pass_omp_oacc_neuter_broadcast::gate): Disable if num_workers is
	1.
	(execute_omp_oacc_neuter_broadcast): Adjust.

Co-Authored-By: Thomas Schwinge <thomas@codesourcery.com>