re PR tree-optimization/78856 (wrong code at -O3 on x86_64-linux-gnu (in both 32-bit and 64-bit modes))

	PR tree-optimizatin/78856
	* tree-ssa-threadupdate.c: Include tree-vectorizer.h.
	(mark_threaded_blocks): Remove code to truncate thread paths that
	cross multiple loop headers.  Instead invalidate the cached loop
	iteration information and handle case of a thread path walking
	into an irreducible region.

	PR tree-optimization/78856
	* gcc.c-torture/execute/pr78856.c: New test.

From-SVN: r244045