Class value_relation was private within the value-relation.cc file.   
This class simply represents a relation between 2 ssa-names, and can 
perform various operations on them.   The oracle uses it under the 
covers to maintains its tables.

It can be used in other places as well, so lets just expose it in the 
header file.

Bootstrapped on x86_64-pc-linux-gnu with no regressions.  Pushed.

Andrew