Avoid fatal fails in predicate::init_from_control_deps

Avoid fatal fails in predicate::init_from_control_deps

When processing USE predicates we can drop from the AND chain,
when procsssing DEF predicates we can drop from the OR chain.  Do
that instead of giving up completely.  This also removes cases
that should never trigger.

	* gimple-predicate-analysis.cc (predicate::init_from_control_deps):
	Assert the guard_bb isn't empty and has more than one successor.
	Drop appropriate parts of the predicate when an edge fails to
	register a predicate.
	(predicate::dump): Dump empty predicate as TRUE.