tree-optimization/112859 - bogus loop distribution

tree-optimization/112859 - bogus loop distribution

When we get a zero distance vector we still have to check for the
situation of a common inner loop with zero distance.  But we can
still allow a zero distance for the loop we distribute
(gcc.dg/tree-ssa/ldist-33.c is such a case).  This is because
zero distances in non-outermost loops are a misrepresentation
of dependence by dependence analysis.

Note that test coverage of loop distribution of loop nests is
very low.

	PR tree-optimization/112859
	PR tree-optimization/115347
	* tree-loop-distribution.cc
	(loop_distribution::pg_add_dependence_edges): For a zero
	distance vector still make sure to not have an inner
	loop with zero distance.

	* gcc.dg/torture/pr112859.c: New testcase.
	* gcc.dg/torture/pr115347.c: Likewise.
	* gcc.dg/tree-ssa/ldist-36.c: Adjust.

(cherry picked from commit 04ba1300407f106a6dd10d346f58a51d87e6d43e)