Add ADT/IntEqClasses.h as a light-weight implementation of EquivalenceClasses.h.

This implementation already exists as ConnectedVNInfoEqClasses in
LiveInterval.cpp, and it seems to be generally useful to have a light-weight way
of forming equivalence classes of small integers.

IntEqClasses doesn't allow enumeration of the elements in a class.

llvm-svn: 122293