Cleanup irange::set.

Cleanup irange::set.

Now that anti-ranges are no more and iranges contain wide_ints instead
of trees, various cleanups are possible.  This is one of a handful of
patches improving the performance of irange::set() which is not on a
hot path, but quite sensitive because it is so pervasive.

gcc/ChangeLog:

	* gimple-range-op.cc (cfn_ffs::fold_range): Use the correct
	precision.
	* gimple-ssa-warn-alloca.cc (alloca_call_type): Use <2> for
	invalid_range, as it is an inverse range.
	* tree-vrp.cc (find_case_label_range): Avoid trees.
	* value-range.cc (irange::irange_set): Delete.
	(irange::irange_set_1bit_anti_range): Delete.
	(irange::irange_set_anti_range): Delete.
	(irange::set): Cleanup.
	* value-range.h (class irange): Remove irange_set,
	irange_set_anti_range, irange_set_1bit_anti_range.
	(irange::set_undefined): Remove set to m_type.