Make irange::intersect(wide_int, wide_int) private.

This method should have been private, and somehow seeped into the API.

Tested and benchmarked on x86-64 Linux.

gcc/ChangeLog:

	* gimple-range-cache.h (non_null_ref::adjust_range): Do not use
	irange::intersect (wide_int, wide_int).
	* gimple-range-fold.cc (adjust_pointer_diff_expr): Same.
	(adjust_imagpart_expr): Same.
	* value-range.h (irange::intersect (wide_int, wide_int)): Make
	private.