Cleanup legacy_union and legacy intersect in value_range.

Cleanup legacy_union and legacy intersect in value_range.

These are cleanups so that multi-range union/intersect doesn't
have to deal with legacy code.  Instead, these should be done in
legacy mode.

gcc/ChangeLog:

	* value-range.cc (irange::legacy_intersect): Only handle
	legacy ranges.
	(irange::legacy_union): Same.
	(irange::union_): When unioning legacy with non-legacy,
	first convert to legacy and do everything in legacy mode.
	(irange::intersect): Same, but for intersect.
	* range-op.cc (range_tests): Adjust for above changes.