With .DEFERRED_INIT ssa_undefined_value_p () can return true for
values we did not visit (because they proved unreachable) but
are not .VN_TOP. Avoid using those as value which, because they
are not visited, are assumed to be defined outside of the region.
PR tree-optimization/112444
* tree-ssa-sccvn.cc (visit_phi): Avoid using not visited
defs as undefined vals.