The base class for ranges is currently value_range_base, which is
rather long and cumbersome.  It also occurs more often than the derived
class of value_range.  To avoid confusion, and save typing, this
patch does a global rename from value_range to value_range_equiv,
and from value_range_base to value_range.

This way, the base class is simply value_range, and the derived
class is value_range_equiv which explicitly states what it does.

From-SVN: r277847