Adjust tree-ssa-dom.c for irange API.

Adjust tree-ssa-dom.c for irange API.

This removes all uses of VR_ANTI_RANGE.

gcc/ChangeLog:

	* tree-ssa-dom.c (simplify_stmt_for_jump_threading): Abstract code out to...
	* tree-vrp.c (find_case_label_range): ...here.  Rewrite for to use irange
	API.
	(simplify_stmt_for_jump_threading): Call find_case_label_range instead of
	duplicating the code in simplify_stmt_for_jump_threading.
	* tree-vrp.h (find_case_label_range): New prototype.