PR middle-end/81897
	* tree-ssa-uninit.c (compute_control_dep_chain): Do not bail on
	basic blocks with a small number of successors.
	(convert_control_dep_chain_into_preds): Improve handling of
	forwarder blocks.
	(dump_predicates): Split apart into...
	(dump_pred_chain): ...here...
	(dump_pred_info): ...and here.
	(can_one_predicate_be_invalidated_p): Add debugging printfs.
	(can_chain_union_be_invalidated_p): Improve check for invalidation
	of paths.
	(uninit_uses_cannot_happen): Avoid unnecessary if
	convert_control_dep_chain_into_preds yielded nothing.

	PR middle-end/81897
	* gcc.dg/uninit-pr81897.c: New test.

From-SVN: r256320