Avoid recomputing the defining predicate chains for the same PHI

Avoid recomputing the defining predicate chains for the same PHI

gcc/ChangeLog:

	* tree-ssa-uninit.c (find_uninit_use): Declare and pass to
	is_use_properly_guarded the variable def_preds.  Free its
	contents before returning.
	(prune_uninit_phi_opnds_in_unrealizable_paths): Same.
	(is_use_properly_guarded): Replace local variable def_preds with
	a parameter.  Adjust accordingly.  Only update *def_preds if it's
	the empty vector.

From-SVN: r226511