re PR tree-optimization/64823 (false "may be used uninitialized", missed jump threading)

re PR tree-optimization/64823 (false "may be used uninitialized", missed jump threading)

	PR tree-optimization/64823
	* tree-vrp.c (identify_jump_threads): Handle blocks with no real
	statements.
	* tree-ssa-threadedge.c (potentially_threadable_block): Allow
	threading through blocks with PHIs, but no statements.
	(thread_through_normal_block): Distinguish between blocks where
	we did not process all the statements and blocks with no statements.

	PR tree-optimization/64823
	* gcc.dg/uninit-20.c: New test.

From-SVN: r220743