tree-optimization/113693 - LC SSA and region VN

The following fixes LC SSA preserving with region VN which was broken
when availability checking was enhanced to treat not visited value
numbers as available.  The following makes sure to honor availability
data we put in place for LC SSA preserving instead.

	PR tree-optimization/113693
	* tree-ssa-sccvn.cc (rpo_elim::eliminate_avail): Honor avail
	data when available.

	* gcc.dg/pr113693.c: New testcase.