tree-optimization/106913 - ICE with -da and -Wuninitialized

The following avoids setting and not clearing an auto_bb_flag
on EXIT_BLOCK which we don't verify for such stale flags but
dump_bb_info still asserts on them.

	PR tree-optimization/106913
	* tree-ssa-uninit.cc (warn_uninitialized_vars): Do not set
	ft_reachable on EXIT_BLOCK.