Cleanups to unsupported_range.

Cleanups to unsupported_range.

Here are some cleanups to unsupported_range so the assignment operator
takes an unsupported_range and behaves like the other ranges.  This
makes subsequent cleanups easier.

gcc/ChangeLog:

	* value-range.cc (unsupported_range::union_): Cast vrange to
	unsupported_range.
	(unsupported_range::intersect): Same.
	(unsupported_range::operator=): Make argument an unsupported_range.
	* value-range.h: New constructor.