openmp: Diagnose non-rectangular loops with invalid steps

THe OpenMP 5 standard requires that if some loop in OpenMP loop nest refers
to some outer loop's iterator variable, then the subtraction of the multiplication
factors for the outer iterator multiplied by the outer increment modulo the
inner increment is 0.  For loops with non-constants in any of these we can't
diagnose it, it would be a task for something like -fsanitize=openmp,
but if all these are constant, we can diagnose it.

2020-07-02  Jakub Jelinek  <jakub@redhat.com>

	* omp-expand.c (expand_omp_for): Diagnose non-rectangular loops with
	invalid steps - ((m2 - m1) * incr_outer) % incr must be 0 in valid
	OpenMP non-rectangular loops.  Use XALLOCAVEC.

	* c-c++-common/gomp/loop-7.c: New test.

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