IntEqClasses: Let join() return the new leader

The new leader is known anyway so we can return it for some micro
optimization in code where it is easy to pass along the result to the
next join().

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@257130 91177308-0d34-0410-b5e6-96231b3b80d8