[PATCH][PR tree-optimization/68198] Avoid CFG explosion due to threading

[PATCH][PR tree-optimization/68198] Avoid CFG explosion due to threading

	PR tree-optimization/68198
	* tree-ssa-threadupdate.c (valid_jump_thread_path): Distinguish
	between threading a multi-way branch and a thread path that contains
	a multi-way branch.  Disallow the case where a path contains a
	multi-way branch and does not thread a multi-way branch.
	(thread_through_all_blocks): Update comment.

        PR tree-optimization/68198
	* gcc.dg/tree-ssa/pr66752-3.c: Update expected output for VRP1.
	* gcc.dg/tree-ssa/pr68198.c: New test.

From-SVN: r230586