tree-optimization/113707 - ICE with VN elimination

The following avoids different avail answers depending on how the
iteration progressed.

	PR tree-optimization/113707
	* tree-ssa-sccvn.cc (rpo_elim::eliminate_avail): After
	checking the avail set treat out-of-region defines as
	available.

	* gcc.dg/torture/pr113707-1.c: New testcase.
	* gcc.dg/torture/pr113707-2.c: Likewise.