Remove uninit_analysis::use_cannot_happen

As written earlier uninit_analysis::use_cannot_happen is duplicate
functionality implemented in a complement way, not adhering to
the idea of disproving a may-uninit use and eventually (I have not
yet found a testcase it helps to avoid false positives) avoiding
false positives because of this or different ways it imposes limits
on the predicate computations.

This patch removes it.

	* gimple-predicate-analysis.h
	(uninit_analysis::use_cannot_happen): Remove.
	* gimple-predicate-analysis.cc (can_be_invalidated_p): Remove.
	(uninit_analysis::use_cannot_happen): Likewise.
	(uninit_analysis::is_use_guarded): Do not call
	use_cannot_happen.
	(dump_predicates): Remove.
	(simple_control_dep_chain): Remove edge overload.