re PR tree-optimization/23386 (bitmap.c is being miscompiled (VRP))

	PR 23386
 	* tree-data-ref.c (estimate_niter_from_size_of_data): When
 	step is negative compute the estimation from init downwards to
 	zero.

	* testsuite/gcc.dg/tree-ssa/pr23386.c: New.

From-SVN: r103106