openmp: Add support for non-rectangular loops in taskloop construct

2020-08-13  Jakub Jelinek  <jakub@redhat.com>

	* gimplify.c (gimplify_omp_taskloop_expr): New function.
	(gimplify_omp_for): Use it.  For OMP_FOR_NON_RECTANGULAR
	loops adjust in outer taskloop the var-outer decls.
	* omp-expand.c (expand_omp_taskloop_for_inner): Handle non-rectangular
	loops.
	(expand_omp_for): Don't reject non-rectangular taskloop.
	* omp-general.c (omp_extract_for_data): Don't assert that
	non-rectangular loops have static schedule, instead treat loop->m1
	or loop->m2 as if loop->n1 or loop->n2 is non-constant.

	* testsuite/libgomp.c/loop-22.c (main): Add some further tests.
	* testsuite/libgomp.c/loop-23.c (main): Likewise.
	* testsuite/libgomp.c/loop-24.c: New test.