Catch 'GIMPLE_DEBUG' misbehavior in OpenACC 'kernels' decomposition [PR100400, PR1038...

Catch 'GIMPLE_DEBUG' misbehavior in OpenACC 'kernels' decomposition [PR100400, PR103836, PR104061]

Actually fixing it is a separate task, but it seems prudent to at least catch
it, and document via a few test cases.

	gcc/
	PR middle-end/100400
	PR middle-end/103836
	PR middle-end/104061
	* omp-oacc-kernels-decompose.cc (decompose_kernels_region_body):
	Catch 'GIMPLE_DEBUG'.
	gcc/testsuite/
	PR middle-end/100400
	PR middle-end/103836
	PR middle-end/104061
	* c-c++-common/goacc/kernels-decompose-pr100400-1-1.c: New.
	* c-c++-common/goacc/kernels-decompose-pr100400-1-2.c: New.
	* c-c++-common/goacc/kernels-decompose-pr100400-1-3.c: New.
	* c-c++-common/goacc/kernels-decompose-pr100400-1-4.c: New.
	* c-c++-common/goacc/kernels-decompose-pr103836-1-1.c: New.
	* c-c++-common/goacc/kernels-decompose-pr103836-1-2.c: New.
	* c-c++-common/goacc/kernels-decompose-pr103836-1-3.c: New.
	* c-c++-common/goacc/kernels-decompose-pr103836-1-4.c: New.
	* c-c++-common/goacc/kernels-decompose-pr104061-1-1.c: New.
	* c-c++-common/goacc/kernels-decompose-pr104061-1-2.c: New.
	* c-c++-common/goacc/kernels-decompose-pr104061-1-3.c: New.
	* c-c++-common/goacc/kernels-decompose-pr104061-1-4.c: New.