make value_range the base class and value_range_equiv the derived class

[Aldy Hernandez]

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.

OK?

Aldy

p.s. There are a few minor cleanups throughout... like moving some 
random variable definitions closer to their first use.  I figured they 
were harmless while I was in the vicinity.
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