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.