[LiveIntervalUnion] Expose extraction of last index in map for external users

[kariddi]

Summary

LiveIntervalUnion exposes IntervalMap's start() method, but doesn't expose the stop() method.

For custom uses of LiveIntervalUnion it is useful.

llvm/include/llvm/CodeGen/LiveIntervalUnion.h

 
   bool empty() const { return Segments.empty(); }
   SlotIndex startIndex() const { return Segments.start(); }
+  SlotIndex endIndex() const { return Segments.stop(); }
 
   // Provide public access to the underlying map to allow overlap iteration.
   using Map = LiveSegments;