openmp: Fix up OpenMP expansion of non-rectangular loops [PR108459]

expand_omp_for_init_counts was using for the case where collapse(2)
inner loop has init expression dependent on non-constant multiple of
the outer iterator and the condition upper bound expression doesn't
depend on the outer iterator fold_unary (NEGATE_EXPR, ...).  This
will just return NULL if it can't be folded, we need fold_build1
instead.

2023-01-19  Jakub Jelinek  <jakub@redhat.com>

	PR middle-end/108459
	* omp-expand.cc (expand_omp_for_init_counts): Use fold_build1 rather
	than fold_unary for NEGATE_EXPR.

	* testsuite/libgomp.c/pr108459.c: New test.