Constify invariant fields of vrange and irange.

Constify invariant fields of vrange and irange.

The discriminator in vrange cannot change after construction,
similarly the number of allocated ranges in an irange.  It's best to
make them constant to avoid invalid changes.

gcc/ChangeLog:

	* value-range.h (class vrange): Make m_discriminator const.
	(class irange): Make m_max_ranges const.  Adjust constructors
	accordingly.
	(class unsupported_range): Construct vrange appropriately.
	(class frange): Same.