openmp: Handle even some combined non-rectangular loops

openmp: Handle even some combined non-rectangular loops

The number of loops computation and logical iteration -> actual iterator values
computations can now be done separately even on composite constructs (though
for triangular loops it would still be more efficient to propagate a few values
through, will handle that incrementally).
simd and taskloop are still unhandled.

2020-08-05  Jakub Jelinek  <jakub@redhat.com>

	* omp-expand.c (expand_omp_for): Don't disallow combined non-rectangular
	loops.

	* testsuite/libgomp.c/loop-22.c: New test.
	* testsuite/libgomp.c/loop-23.c: New test.